Anyways, I think that professional sports exists for the same reason that movies exists; it gives people a form of entertainment to watch. It's really an amazing thing the way people identify themselves with a given team and loathe those who don't identify with the same team. Soccer riots? Oh man, amazing study of human psychology. But I digress.
Sports does provide an outlet for tension for people. I personally love baseball, and interestingly enough whenever I was pissed off or stressed, for some reason going out and playing some baseball calmed me down and get my thoughts straight. Sports are obviously also a way to stay fit which gears towards reducing health problems.
I had some overlaying point in this post, but I have seem to have lost it now. Ah well. Sports have existed for a long time, and I don't see there being an end to it any time soon.