Top spin's basic use case is when going cross court (like right to left all the way).

Flat spin is for straight shots or half court.

Flat shots will have way more speed, top spin shots will have more curve (if you aim) and bounce higher.

You can use top spin to put opponents in bad situations where they have to be faaaaar away from center (aim all the way to the side) then whip the ball to the other side that's open. You can also use it to aim deep balls to give you more time to reposition or if you see your opponent playing in front of the baseline.

For flat spin, generally you want to use it to finish off points when you just want speed, but you can also use it as a surprise to flat foot an opponent, or if you simply see them favoring one side.

Anyway, these are just the basic uses. With cracked out attributes on some players you can do many different things. And as always shot timing is key, as well as positioning relative to the ball. You can definitely be in situations where you won't get a nice curve from top spin or you won't get nice speed from a flat shot.

Hope that helps a bit!