People are pretty violent. We need an outlet, because it's in our nature. Giving us a virtual release is a sensible way to decrease our violent outbursts. In the absence of hunting our own food, killing buffalo with a rock or warring with another local tribe, we're going crazy. It's part of the reason why psychological disorders are so common in the West, when Kenyan tribesmen are doing just fine. I'm sure our western problems are awful and all that, but they're not exactly starving to death or getting murdered by a warlord. The trick is to let it out, and fantasy of all kinds helps.
P.S. Ever noticed that, when you wreck things, if it's too hard or soft, it's unsatisfying? That's because the perfect grabbing, tearing, punching sensation is flesh