well its always a combination of the cpu and the gpu that makes the final image. but in cases like HD video, you need a lot of processing power to make that image happen. in motherboards like this one, the 6100 video chipset is not powerful enough to process a 1080p video stream... so it would have to rely on the cpu to do the crunching. then, the gpu can take that processed video and overlay some subtle enhancements on it before it goes to the monitor. thats how you get a good HD output normally.
but nowadays we have gpu's that are fast enough to process and enhance the video all on their own.... leaving the cpu almost entirely out of the stream. its just another way to do it, it doesnt mean its better or worse then having the cpu help, but since this all started with cpu's doing all the work you get the best compatibly across all platforms when you have a cpu that can handle its own. of course, gpu accelerated video is very common as of the last couple years, so its not even much of an issue anymore.