You want, or can only reasonably get, a physical copy, and the game is expected to be available in limited quantities or popular enough to sell out in some places on day one. By "reasonably get", I mostly am talking about the fact that some people don't have good enough Internet to download most modern games.
Various games and companies will also run some preorder bonuses, which provides some financial incentive to preorder.
Normally, I'd imagine people preorder if they plan on getting the game Day 1 already. Unless I'm mistaken, there's generally a way to back out if the game turns out to be awful, such as refunds (Origin, Steam, and GOG all offer them) or cancelling the preorder.
Various games and companies will also run some preorder bonuses, which provides some financial incentive to preorder.
Normally, I'd imagine people preorder if they plan on getting the game Day 1 already. Unless I'm mistaken, there's generally a way to back out if the game turns out to be awful, such as refunds (Origin, Steam, and GOG all offer them) or cancelling the preorder.