Are game jams mainly for practice?
Up to you. With limited duration, game jams are good way to train discipline or at least time management.
However, no one could prevent any dev joining in a game jam to have other reasons.
What I mean is, has any game jam gone on to become a professional game?
Yes, some have became pro game. Here's an example : Roller Dome . The pro game is : Roller Drome .
There's also Hungry Knight who became Hollow Knight.