I'll use an RTS as an example.
Your GPU, takes and renders and image to your screen, the better your gpu the higher the visual settings and faster the frames can render increaseing your frame rate.
Your cpu must do everything else, this includes physics, calculating environmental effects(gpu still renders them visually), calculating damage, keeping track of build times/ques, tracking all unit stats, path finding for units to calculate quickest way to move, all aspects of artificial intelligence, game timer, win/loss conditions, as you can see the list just goes on.