Which actually gives you more room to click in and less wasted space. Memorize that, if you haven't already, and you'll be fine. And don't worry, if you accidentally hit minimize the genie will show you where in the dock the window went. Then the middle button will become your favorite of all. A round object is geometrically easier to hit.
Rounder is more organic.
More comfy feeling. Good Human Interface. That's because if you turn a square diagonally it will fall through the square hole. A circle won't. More usable space with a round button.