Because many newer interfaces require texturing and rendering capabilities that 3D cards have. The same is True of Vista's / Windows 7's Aero and Mac OSX. If you want to do anything fancy and retain decent performance you need to do it on the GPU. Otherwise you are requiring the CPU to do too much and performance will suffer terribly.
A 3D GPU is massively parallel. Which means drawing a screen, manipulating it and redrawing it again takes little effort and memory.
If you want a good place to start additional research, look at Microsoft's DWM. It gives a lot of information for what these interfaces do differently than the older CPU line, by line renders.