It helps me think through my ideas and acts as a litmus test for my opinions. If there's a strong argument that I find it difficult to respond to, then I either have to accept that argument or work out why it's wrong. I think it's healthy to have my opinions constantly challenged, because it avoids the sort of stagnation of ideas that you get in closed communities or internet hugboxes.
I'm also pretty competitive and enjoy winning debates, but only if the competition's strong. Defeating strong opponents is much more satisfying than beating weaker ones.