Nothing. Your CPU can do everything your GPU can. At like, 1:100000000th the speed. Do you want to play your games at about 1 frame every 3 minutes? Then use your CPU.

Then why have a CPU? Why not just a GPU? Because to make the GPU do the type of math that is needed to draw images very very very very fast, it has to be made poopy at other kinds of math. This isn't a software problem either, it's a hardware problem. IE the little squiggles we etch into crystals to zot electricity through to make the crystal think for us need to be drawn in certain ways to work how we want, and we can't have two doodles overlapping each other.

A CPU can do almost anything, but it's slow because of it. A GPU can do graphics (and certain types of AI work) very very very fast, but can't do anything else quickly (or in some cases, at all). So you need both. Unless you don't give a hoot about fancy graphics and are okay with your computer being able to produce graphics that are on par with 80s and VERY early 90s PC graphics only.