Well, excluding all the good things which come with just participating, HeartBeast will play and showcase your game on his YouTube channel, that's a guaranteed thousands of views. That'll for sure help with exposure alongside getting to say "I won a game jam which had over 500 participants" for future opportunities.
I'll be playing my favorite games on my channel. As for the highest scoring games (in the voting) the prize is really just bragging rights. The jam is more for fun than an actual competition. It is a great way to get feedback and practice your skills!