Most power in a computer is spent on a bus. Most power within CPU is spent on internal bus and memory access. This interconnect between framebuffer and accelera

Most power in a computer is spent on a bus. Most power within CPU is spent on internal bus and memory access. This interconnect between framebuffer and accelerator would limit the speed and eat lots of power, unless the framebuffer would be replicated by GPU during rendering. That is why GPUs normally have a small part that serves as framebuffer, but memory bus is shared with the accelerator that needs it most. Cheaper to reuse the expensive part.