I expressed myself wrong, "good on the resume" include portfolio too in my mind.
For the learning part I guess you're right but it would mostly be about rules and deadline I think. I don't even know if you get some kind of constructive feedback when you loose (I assume you don't) so you really don't learn much on sound design. If what I did is bad I loose but I don't really know what was bad.
Contacts tho I don't see how I could make any in a sound design contest since I would do it alone. I would maybe get contacts if I win yes but not for just being in the contest.
I am talking only about sound design contests, not game jams and stuff where you need a team.
I might be completly wrong but that's how I feel about it right now.