Comment by peterfirefly

Modern "GPUs" have the ability to scan out from multiple buffers at the same time and do rectangular compositing during the scan out -- basically like a few giant sprites. It's got absolutely nothing to do with normal GPU work. Zero.

It lets a few unobstructed windows do animation without having to involve the compositor => lower latency AND lower energy use. They do NOT have to be full screen at all.

Oh, and they can also handle framebuffer formats that aren't RGB. This is very useful for video, as the natural format there is YUV or something similar. The YUV => RGB conversion step becomes unnecessary.