Are game jams mainly for practice?  What I mean is, has any game jam gone on to become a professional game?  Thanks

You retain all rights to a game you've made, unless you sign a publishing contact or similar. Taking part in a jam doesn't change that.

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.

It depends on the jam.

There are jams for learning, jams for fun, jams with prizes, small jams, huge jams and so on.

Ludum Dare is rather big.

Game jam games can become "professional" games, but that is very rare. Like, really rare. And in my opinion it is not because of jam fame, but because a developer took the jam version and developed that prototype further.

A famous example is Baba is You.