Computers don't "need" GPUs. It's just that if you have the CPU doing all of the processing for images then there is a whole lot less CPU "time" available to do all the general-purpose stuff a computer does and everything would be slower - including the graphics. GPUs are designed to mathematical processing very quickly and can do graphics processing while the CPU is doing other general-purpose stuff. There are lots of chips on a motherboard doing special purpose stuff so that the CPU doesn't have to do it (that's why phones now have SoC - they put a bunch of special purpose shit on the same die as the CPU).