When I'm rendering a scene in GPU mode, blender becomes very CPU & GPU intensive. This is most noticeable in cycles, luxrender tends to be less CPU intensive (in openCL mode). I would like a list of tasks the CPU performs when i'm in GPU mode.
It has to calculate and bake the frames when you are rendering... Try to think of the GPU doing the huge graphic tracing, and the CPU baking the elements in the scene, like keyframes for example... The CPU also is the one who adds the frames to the final file... Hope this helped! :)