Computers don't need GPUs. Older computers from the 80s sometimes didn't have any GPU, and the CPU was responsible for redrawing what you see on the monitor.

The problem with that is that it costs a lot of CPU time to do so, redrawing all the pixels 50 or 60 times per second.

GPUs started out as nothing more than a helper chip(set) for the CPU, so it wouldn't be doing the pixel pushing, but could do other stuff at the same time.

As what we wanted to see on the screen became more complex GPUs consequently also became more complex. First it was 2D acceleration to improve drawing windows and sprites on the screen, later 3D acceleration for obvious uses.

Or said in one line, CPUs are generalists so they can do the 'anything' computers are known for, GPUs are specialists so the CPU can continue doing generalist stuff, like instructing the GPU to 'draw a rectangle there', 'move the sprite from here to here over X frames', or 'add a sphere to the 3D scene'.