The nice thing about compositing window managers like compiz is that 1. they offload the responsibilities of the window manager from the cpu to the gpu (stated above by t6) but 2. they offer true window shadows etc, and the ability for a 3d desktop. Most people see the 3d desktop in the form of the effects, however with new compositing and desktop effects it completely changes the computing traditional desktop metaphor. 3. the other nice thing about compositing window managers was that it ushered in a new way of computing, we now have gpu accelerated apps, cuda, and with OpenCL the gpu can be spoken to within the operating system without having to sidestep to another application to do so. The ramifications of this technology is usually only seen skin-deep (the 3d effects) but in fact there is more to it than that.