It's something to do when women are bothering us.
But seriously, it's entertainment. The most fun I've had over the past few months was getting together with the few males in my family, and playing Halo.
As for why you would get the new system. Well, if you never got anything upgraded, we would all still be using smoke signals.

But getting a new system is kind of like getting a new car, if you know what I mean. It's better than what you had last. I'm sure others here could emphasize on that.
And the systems are not similar. I mean, from what I've seen, the PS3's graphics blow the 360's out of the water. Thats just one example.