There's nothing wrong with watching sports. It's just another form of entertainment, that's all. It can also bring people together with a common sense of community, from rooting for your local high school team all the way up to rooting for your country's athletes in the olympics.
Sure, some people take it too seriously, just as some take other forms of entertainment too seriously. In fact, some people can be downright jerks about it. But I think most sports fans know, as I do, that the outcome of a game or a season isn't going to change their lives one bit. It's just more fun when your team wins. There's nothing wrong with being proud of someone else's accomplishments, or feeling bad when someone loses.
As for determining the outcome of a sporting event, it happens all the time. It's called home field, home court, or home ice advantage. Sure, the guy sitting on his couch eating Fritos and watching a game won't have any effect, but even a few hundred people in a small high school gym, all cheering for the same team, can have a big say in the outcome of a close game.
Yeah I know all about the bad things connected with sports. Cheating, steroids, gambling, greed, even violence. But there's also a lot of good that comes from it, both for the fans and especially the athletes. The world would be a much more boring place without it, IMO.