Yeah, but you can do things to each transient with a compressor changing the character of each hit as well as changing the balance between them (if that makes sense) so you might want to compress the drums even if you are triggering them all at the same velocity.

You can change a sloppy open hat into something that sounds like a tight clicky thing if you want to with a compressor.

Sometimes we put volume changes into a part after the compression to bring back dynamics whilst still letting the sound retain the same character. You need to be careful using velocity for anything when you are running alot of processing as the sound will change character as the velocity changes which may not be what you intend.