What would be the difference between writing to shared hardware pixels over a shared memory performance wise? I mean, pixels are just like a memory except that

What would be the difference between writing to shared hardware pixels over a shared memory performance wise? I mean, pixels are just like a memory except that they glow. They hold their value and can be writen a new value in sync with a clock (which is normally 60 Hz for most monitors which is MUCH slower then on chip memory). There could be no performance benefit.