From Apple’s website. When discussing the new fast user switching in the next version of Mac OS X, they say:

Because we can
Mac OS X animates transitions from one user to another. The current desktop becomes a texture placed on a 3D cube that rotates out of view while the incoming account desktop rotates into view on another side of the cube.

I love that header: Because we can. What better reason do they need? I love being an Apple user! :) The only thing that could make it more enjoyable would be Doug Wilson calling it “anabaptistic”.