You can ask "Can't you ___?" instead of "Can you ___?" when you're not just asking a question, but trying to convince someone to do something.

Imagine that you're trying to convince your son, who's playing a noisy game, to play in another room so that it doesn't distract you. You can say:

Can't you do that in the other room?

You can sound either polite or annoyed when you say "Can't you ___?" But in any case, you are trying to convince the listener.