Hello, I have the feeling its a lot of wasted time. What could be the benefit of them, traffic, notoriety ? Regards Thibaut
It can't be termed as a total waste of time and effort. Not everybody who participates in an SEO contest participates to win. Some of them look at it as a platform to try out new ideas, and they can see what Google feels about it. The prize is a big motivation, and if you do win, people will know your name and come to you as clients. For the organizer, it is publicity + backlinks.
Trying out something new and testing the skills is a refreshing experience. Yes, sometime I feel that it is a waste of time. It all depends on the theme of contest.