It doesn't need to. It's typical to do this these days, but they could still arrange all of the pixels on the CPU and then blit it onto the screen. There's an API to do so in every major OS.

Since it's more than quick enough to do this on the CPU, they're likely doing it for things like animations and very high quality font rendering. There's image-processing going on when you really care about quality; oversampling and filtering.

I suspect one could do most everything Zed does without a GPU, but about 10 to 20% uglier, depending on how discerning the user is on such things.