Because some people really like it. I always feel claustrophobic in tiling window managers, for example.
There is a significant geek population that likes tiling WMs. More power to 'em. But I think the general public probably like the freedom to overlap windows as they see fit.
My immediate response is that yes, there is, because it preserves the physical feeling that I have. That is to say, my windows go where I want them to go. That's control that I like.
The other response is that there is because you can hide things you aren't using behind other windows. Right now, the 4 windows I have open wouldn't all fit on a single screen without overlapping.
