I know most of us do this for a hobby, but we would all like our art to be seen. And someone mentioned that if you are part of a small community you can rack up 50 comments in no time. So the rating system, I thought was a way of trying to get the good stuff to the top of the pile. I have found that I find most gems when looking through peoples "favourite image" section, a truer but not perfect way of seeing the good stuff.
But then again this points me in the direction of my tastes in art, which isn't a bad thing!