Do you ever have any restrictions on race or class in the games.you run?
Broadly, if I own the book it is in, it is fair game. If I don't own it, we can discuss - if you have a cool idea that isn't just about mechanical power, and I have the funds, I will often buy the book. I have, on occasion, limited a particular race or class for balance reasons. It has been many, many years since I put in a restriction for worldbuilding reasons.
As noted upthread - I generally build the game around the player choices, not restrict the player choices to fit into my world - if the player wants to play a tortle paladin, it will be a world with tortles and paladins in it - I'll talk/negotiate with the player about how tortles and paladins fit into the world.
If I, personally, want to build my own cool stuff into the world, I do that around the player's choices - you can think of it as "Yes, and..." cooperative worldbuilding. My passion for running a game does not come from realizing my game world, but comes from the player's experiences playing - if the players are getting a good experience, I'm a happy GM.
If so, what's an example and what do the players get for it?
So, as above, what the players get is... basically what they want. Maybe with some negotiation.
I am not a "one game, one game world" kind of person, and I haven't been since 1986, and the release of the old FASERIP Marvel Superheroes game. D&D is great, I love it. But there's a lot of other game experiences to be had. So, broadly, it is safe to say that, each RPG campaign I run is not just a new game world, but is a different ruleset - one in Deadlands, the next Ashen Stars, then D&D, then Spirit of the Century.
Luckily for me, the players I typically work with are similar - they
don't want to play several campaigns in the same system and same world. So, I don't run that. Once I have dispensed with the idea that my campaign world is a persistent, long-lived thing from which I gain pride and passion, the rest comes naturally. I don't feel a need to
own the thing.