A unique thing about tabletop roleplaying games is that when you buy them, you're mostly just buying rules. Some rulebooks also describe in-game items, and some even come with a sample adventure tacked on at the end, but the thing you carry from game to game is a book on how to play, not what to play.
This model was created by the world's first roleplaying game, D&D. Historically, you were expected to sketch out your own gameboard, call it a "dungeon", and then apply the rules to it. On the inside cover of the 1st Edition of D&D, there was even a legend of dungeon map symbols so you knew how to annotate your gameboard.
Today, game masters still sketch out their dungeons on dry-erase boards or on virtual tabletop software.
Building your own adventure is one of the most empowering aspects of tabletop RPG. All you really need are rules and maybe some random dungeon tables, and you've got an infinite number of games. In addition to that infinite number, I've got five good reasons you should be designing your own adventure for your favourite RPG.
1. Zero preparation
No exaggeration, building your own adventure requires no preparation. You can build an adventure as you play. Tell your players they're outside of an ancient tomb that they've heard contains untold wealth, or a deep space mine, or a cyberpunk office space, or a post-apocalyptic bunker.
Once they go inside, draw a room on a scrap of paper, add two closed doors, and describe some random items in the room. Repeat this for each room they enter, adding in some interesting items, aggressive monsters, interesting characters, to mix things up.
I do this at least once for every gaming convention I attend, when a game master is suddenly unable to make it to their session, or when there's a group of latecomers with no game to play.
Spontaneous games of D&D are a superpower.
2. Zero commitment
There are many times when I want to play D&D exactly one time. Finding a good gaming group is a little like assembling a rock band. You need the right people, because you're signing up to meet with them every week for a few hours. If you don't get on, that's not going to be fun.
So when a group of strangers get together to play a game, I like to play a short adventure, from start to finish, so that we all have the guilt-free opportunity to opt out of future games if we need to.
Other times, there's a group of players who have no intention of playing together again in the future. An RPG shouldn't require a long-term commitment to play. You don't invite somebody to a game of Monopoly with the subtext that they have to meet with you for a weekly game of Monopoly, until death do you part. Sometimes people just want one game of D&D, and that's OK.
When you build your own spontaneous adventure, you can end it at any point. It's just a game, nothing's written in stone, so the great treasure or big bad evil monster can show up whenever it's most convenient.
3. Zero cost
Once you've purchased a board game, you own that board game. Expansion packs notwithstanding, there are no further purchases. You own it, you can play it.
An RPG is no different. You bought the rulebook, maybe several of your friends have bought the rulebook, and there don't have to be further purchases. Design your own dungeon, apply the rules.
4. Everybody is a player
With long, complex published adventures, it's common for one person to play as game master because that one person has read and internalised the adventure, and is actively tracking all the different components in play. In a campaign I ran from level 1 to 20 over the course of two years, there was no scripted villain because I was assembling it from several adventures and homebrew content. Eventually a villain arose from the story, but only because I kept bringing a specific NPC back into the game. She wasn't the main villain originally, she wasn't even a villain at first, but the players expressed distrust and the more I brought her back the more they demonized her, so by the end of it all she was involved in a scheme to summon Tiamat. That probably wouldn't have happened if my gaming group had switched out game masters every game, or every adventure. It's hard to maintain through-lines without a single author.
But that kind of thing is really only an expectation when there's a concept of a "campaign". If you focus on your own character instead, ignoring the "story" of the world, then you don't need through-lines or conspiracies or intricate plots. As long as your character levels up after 1000 experience points, or gets a new skill after they earn karma, or whatever milestone you're looking for, that's all the story you need.
When you uncouple the game experience from the story, you no longer need a single game master to keep track of the game world. Every session, a different player can be game master. They bring the dungeon, they run the dungeon, everybody wins (or dies, depending.)
5. Casual gaming
I've met people who genuinely think that an RPG takes 6 hours to play, and that's not including building a character. I think some people have this impression because a lot of gamers spend that much time on their RPG. I don't blame them. It's fun.
But I also have a short attention span, I guess, because I play for 2 hours. Admittedly, I play several times a week, but to me a 2 hour session puts pressure on the group to focus on the game, and we usually get more done in 2 hours than I do in longer games filled with socializing and indecision.
My point is, D&D doesn't have to be a ritualistic undertaking. You can play an RPG over a long lunchbreak, in the evening after dinner before bed, on a ferry ride, or whenever suits you. If you have time for a game of Clue, you have time for D&D, and one of those two games is better than the other.
Try it
Those are five good reasons. See my separate blog post for five good reasons to play officially sanctioned adventure instead (this site isn't called "mixed signals" for nothing.)