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.