Kids have shorter attention spans. Having recess is healthy for them. It gives them a chance to let loose and get all their wiggles out. Take away recess and we are going to have more problems in class with kids who aren't paying attention or able to focus.
Kids need the socialization that recess provides. Most jobs allow for at least one 15-minute break in the course of the day, why shouldn't a school day? Kids are there all day long working. They should be allowed to have a few minutes each day to get out from under.
Recess should be an important part of the day for kids, especially in elementary school. I have a 7 year old grandson who is a giant ball of energy. They need to release that energy instead of sitting still all day. I think it's more important for the teachers than it is for the kids. The poor teachers must go nuts trying to keep their attention.