In the old days, the CPU did everyhting, then earl;y GPU's released that weren't great but helped the CPU with high powered games (like Quake, Unreal, etc). As time went buy, GPU's become more and more powerful, and took more and more stress away from CPU's. Thats why you will sometimes see someone have a 1080ti paired with an old I5 or I7, because the GPU does most of the graphics rendering now. CPU's are more for physics and AI, and helping with framerate.
A GPU is the 'engine', the CPU is the car driver.