This is more for having fun and know other game developers. Who knows maybe a game created in one of the mini jams could be your next big game, guess that’s the best prize anyone could ask really! 

The prize is knowledge and experience; learning many things during the jam, gaining friends and contacts and having fun with experimenting!

It's also possible that you indeed find your favorite and most successful game project out of the jam entry. It could even make you a billionaire some day!

Even if you can't finish your game during the game jam, there's always something you gain from it, so it's worth it even if you fail.

Maybe it's even a good thing to fail, if you can see what went wrong and how you could grow as a developer in the future!