I'm a big fan of the alignment system in D&D, and have been since I learned about it in the original DragonLance Adventures book. I used to think that there could be nothing to lure me away from it, especially not in the context of D&D. I'll admit that recently there's been a system that has appealed to me as a valid alternative, but I'm nevertheless still a fan of alignment. I'm such a fan that I'm happy to defend it, and explain it to people who maybe haven't given it too much thought (or possibly too much thought.)
First, I may as well identify what people have against alignment. It's only because some people have a complaint against the alignment system that I thought to write this post. Because I myself have no problem with alignment, though, it's possible that I don't fully understand the complaints people do have. So I'm going to explain what I believe people dislike about alignment so that there's context to my apologetics.
You might view the alignment system as a kind of script in an otherwise freeform game. You may feel that because you chose Chaotic Good as your character's alignment, you can never ever agree with city guards because they represent Law, and you're not Lawful. Or maybe you've chosen Lawful Good, and so now you can't steal a loaf of bread even though you're starving.
You might view the alignment system as inherently judgemental. When you encounter an orc, you want to talk, maybe share ideas, and generally assess the orc on a personal level. You do not want the "secret code" of alignment dictating to you that the only logical action is to kill the orc instantly because it's "evil".
I see these complaints as completely valid, so much so that they're some of my arguments in favour of alignment, although I have a different spin on them.
D&D is a game, and the main source of conflict in D&D's rules is pretend physical violence. There are other kinds of conflict, and they're very entertaining. There are dungeon traps, puzzles, chases, races, and even verbal debate. Paizo, prolific as they are, has even published a 200+ page about social intrigue, and there are literal rules involving dice rolling and strategic choices to fight with words rather than sword.
However, many players enjoy the thrill of combat, and so there does tend to be a lot of pretend killing in a D&D game.
It's not much fun to take time after each game to justify why your player character can still be classified as a "hero" after doing nothing but slaughtering living creatures for the entire game session. After all, you wouldn't be killing at all, if it weren't in the rulebook. The game sets you up for moral failure.
You know your player character is good, or neutral at worst, because the game world tells you so, despite the things your PC does that you would never do in real life. You have a Good or Neutral God that not only absolves you of your bad behaviour, it even encourages it.
Even the infamous evil campaigns benefit from alignment. Yes, your player character does horrible things and gets XP for it, but in the topsy-turvy game world, the poles of your moral compass got reversed, and so every selfish and evil act your PC makes is because you follow the straight and narrow path of anti-righteousness.
Hypothesis: It's your game's job to provide you with absolution, and alignment is the way it does that.
In a make-believe fantasy world, nothing you do actually matters. While on one hand it can be liberating, it can also be confounding. D&D is an open ended game on a good day, and other times it's barely even a game. Any player who's ever spent eight (or more, sometimes a lot more) hours wandering aimlessly through a mega-dungeon knows how disorienting it can be to play in a game demanding that the players create their own goals.
It largely depends on a gaming group's style of play, and the DM can often help guide players in the direction of the actual plot (assuming there is one). But sometimes there just isn't a whole lot to latch onto.
Alignment can solve that riddle. If your player character's eager to do the works dictated by their alignment, then when all else fails you at least have that goal. And that's enough to carry you through any number of sessions. Ask any Paladin or Cleric.
If you don't want to be told how to act, then you're Neutral. In fact, I think true Neutral is the hardest alignment to play, because only situations that directly effect you ought to elicit a response.
Hypothesis: Sometimes all a player has to go on is the pursuit of their alignment.
I don't [mind the metagame](https://mixedsignals.ml/games/blog/blog_metagame), and so I think it's pretty silly when a gaming group gets hung up on the debate over what a player would do and what a player character should do. It can happen in any number of situations. An obvious trap that seems like it would be really fun to experience, but would be stupid for a character to trigger. An NPC demanding aid from player characters when obviously just hiring a specialist would be a better choice. Whatever obstacle stands between the players and the plot is an opportunity for players to step out of the game, and admit "I think I have to do this, because it's obviously the next part of the adventure, but WHY would my character do it?"
You might think that there are actually very elegant ways around this. The DM ought to provide in-game incentives. Or the DM could have an external force compel the players to do something against their better judgement. But you know what? that's exactly what alignment is.
Alignment is the excuse your players need for their characters to do something that either the player or the character doesn't think is the best course of action. It's why I don't allow evil characters in my campaigns: I expect players to have their characters act against the table's better judgment because it's what the character's alignment compels them to do. Chaotic and Lawful are powerful plot devices. They can propel the story into directions nobody ever predicted.
Hypothesis: Acting according to character alignment liberates players to advance the story.
This one is purely mechanical, and it might be new to you if you never played AD&D, D&D 3.5, or Pathfinder. Previous editions of D&D had spells that affected Good-aligned creatures only, or Bad-aligned creatures only, and even Lawful-aligned creatures or Chaotic-aligned creatures. Alignment was an attribute, a sort of tag, you could stick on players and NPCs and monsters, and it changed the way the game world worked. An adventuring group could walk into a trap that only affected two or three members of the party, and at first the players would guess maybe it was the PC's ancestry, or maybe the weapons they carried, or their class, or their physical placement within the room. And then eventually, finally, somebody would realize the magic was only affecting the Chaotic-aligned characters, and the puzzle was solved.
Spells could detect alignment, too. While 5e's Detect Evil and Good spell just decects fey, aberrations, celestials, elementals, fiends, and undead, there used to be a spell literally called Detect alignment that just told you the alignment (in the form of an aura) of your target.
This was also a way for alignment to actually matter in the game. When your moral code affected how magic interacted with you, you were inspired to really think about your alignment choice.
Hypothesis: Alignment is a useful tool for game mechanics.
Some players need encouragement to play costructively. An in-game morality system removes the babysitting aspect from being a Dungeon Master, and makes good behaviour a goal of the game. I appreciate this, because as a DM I have no interest in playing the role of a school teacher. I don't want to be the diplomat or the disciplinarian, at least no more than anyone has to be in any given social situation.
Alignment gives the DM a game device to reinforce a positive table culture. Is a player being selfish? trying to initiate PvP? causing trouble for the party? When there are clearly defined rules around alignment, it's easy to explain that either a PC is not acting within their alignment, or why given their alignment it makes no sense for the rest of the adventuring party to engage with them. It's also easy to bring down the wrath of a significant God or Power who has a vested interest in preserving the dominant alignment of the region or of the adventuring party.
Of course you can do all that without alignment, but then it can start to feel pretty judgmental. It starts to seem that anything the DM doesn't like is forbidden. But with a clear system of alignment, you can practically cite fantasy scripture to explain why a player character needs to start behaving, or else be kicked out of the party or smited or whatever.
Hypothesis: Alignment is an in-game tool for helping problem players align with the rest of the group.
The planes of existence are structured around alignment. Because the planes of existence are imaginary constructs, this doesn't really matter. You can remove alignment and just declare that the planes work the way they work just because. But really, the planes were developed around alignment.
The Gods and Powers and Fiends and Undead, and more, are structured along the lines of the alignment system. There's consistency because of there are rules and definitions. Player characters know what plane they'll go to after death due to their alignment. That can save a PC's life. Trust me, I've seen it happen.
I'm dangerously close to a Planescape-style debate here, but alignment gives structure to the fantasy cosmology of D&D in much the same way that astrology and magic crystals give structure to mysticism in the real world, and the way that ancient myths and legends give structure to modern religions.
Hypothesis: Alignment is to D&D what a Cartesian graph is to Mathematics.
At one time in my life, I really did believe that alignment was essential to D&D. I've seen the game evolve, though, and I see now that it's not strictly essential. You can do without it, and the game still works. I acknowledge that.
However, I do believe the game is better for its alignment system. And interestingly, the strongest argument for it, for me, was my final point. The one that's actually pure flavour. I play heavily into the D&D cosmology, partly because of (or against?) my upbringing, and partly because I'm just a sucker for mythology. But when you start dabbling with morality, it's essential to have a code of conduct, just as it is in real life. You have to have a social contract that defines specific consequences for certain behaviour. Player characters are free to opt out. You can be Neutral in D&D. You can ignore morality. But the rules are defined, and so when people poke at the boundaries, it's no surprise when imaginary external forces poke back.
I'm not much of a believer in the one-size-fits-all theory, so I see how alignment doesn't fit into every gaming group's sensibilities or preferences. Hopefully this post has explained what's useful about alignment. I discuss my favourite alternative to alignment in a separate post.