You can never have too many spells. That's what they say. And I guess they're right, because it seems I never tire of looking through spells.
Even if I never have the occasion to use a spell, reading a spell is like reading a story that has yet to be written. In order to understand a spell, you have to place yourself in an imaginary situation when the spell would be useful, and you have to imagine what effect the spell would have upon the world of this imaginary game. And so, if you let it, your mind runs wild with all the possibility wrapped up in a spell.
It's the briefest fantasy. It probably only lasts as long as it takes you to read the text on the page. But it's evocative, and a lot of fun while it lasts.
It makes no sense, but the "same old" spells from the Player's Handbook get old, even though you probably haven't used half of them. As a player, you want more spells to choose from because having more spells makes you feel wealthy. You take comfort in knowing that no matter what situation arises, you'll have the perfect spell for it. It's a security blanket, and it's empowering.
Better yet, new spells surprise and delight your other players. Everyone's heard of Speak with plants but who's ever heard of Speak with object? Everyone knows Thunderwave but who knows Sonic Boom? New spells add variety to the game, and everyone wins when fresh new ideas are introduced to the game.
The nice thing about a good spell is that it makes ingenuity a byproduct of game play. We all know that a player who "discovers" a new spell didn't write the spell, or even literally discover it, because it's in a printed book being sold on shelves all over the world. But that doesn't change the spell's effect on everyone at the table. Finding a fun new spell, or the perfect spell for a once-in-a-lifetime situation, or just throwing out a wildcard that inspires a new strategy for the group, makes the player casting the spell look and feel like a legend.
And for everyone else at the table, the spell is fun to witness in action. No matter what the effect, the players can revel in the fact that their characters are pals with that caster; the one with the cool spell.
Independent and third-party content is important. It was always important, but after the OGL landed, it became essential to growing the hobby. I embrace third-party content as much as I do official game content. In fact, there's more third-party content out there than official content, since third parties are legion.
Whether your source was Dragon magazine, Dungeon magazine, Trollzine, Polyhedron, Pathways, or some other zine, finding obscure lore and rule variants and game tips has always been a part of D&D tradition. Spells are part of that tradition, and it's a good thing they are, because rummaging through books and zines for cool spells is a great way to get new ideas, a great way to come up with new custom content, and a great way to play D&D while not playing D&D.
As a kid getting tangentially introduced to D&D and Magic The Gathering by watching my friends play, there wasn't really an awareness of the concept of different fictional universes [franchises]. My friends and I didn't understand or care that the map of Middle Earth was separate from the map of the Forgotten Realms. We didn't understand or care that the Voice from Dune (and the Weirding module of the movie) didn't exist within D&D because the rules for those ideas weren't written down anywhere in a source book. Heck, we didn't even care that Godzilla and Star Wars weren't in the same universe, so we certainly weren't going to let a difference in game rules stop us from using spells and items from Zelda or Shadowrun or the Realms in a Dragonlance game.
The result was, as it still is now, more fun had by all. And that's why we play games, so get back to your research. You have spells to uncover.