TLDR: the gpu is specialized in tasks needed for rendering. That way the cpu doesn’t have to do all the work and can do other tasks instead that the gpu isn’t capable of doing.

My best eli5:

You have two workers. One can do everything but not particularly fast. The other can draw really good but can’t do anything else.

Of course you could have the first worker do everything, but that would be slow. Instead you have the first one do all the story and background, and the second guy just for drawings, so that the first has time For other stuff

A bit more technical:

In a really rudimentary CPU you have a component that adds things together. Now let’s say you want to multiply two numbers. You could do that by simply adding several times.

If you do that you block the component that adds for quite a while. So instead you make a new component dedicated to multiplying. Now you can simultaneously add something while something else is being multiplied.

So you’re dedicating a component to a specific task in order to have more performance. And this goes way further. Anything you gpu does your cpu can too. But it’s like the multiplying: if the cpu does everything the gpu does Them is occupied a lot and can’t work on other tasks.

The GPU has components for tasks that are very common for rendering. So you take the entire workload of rendering an image away from the cpu and shove it onto the gpu. Now your cpu is free to do other things that the gpu can’t.

So yes, you can run a computer with just a cpu. But this cpu would constantly be occupied with rendering, and while its doing that it can’t do other stuff. You end up with a lot less performance.