It's useful on slower connections, if you're trying to locate and move to an area of a video that's already loaded, without aimlessly clicking around.
It also sort of acts as a bookmark, to know what you've watched. Sometimes I'll open a long video (1 hour or more), and skip around watching a few parts for 30 seconds here and there. I then realize I want to return to one of the parts I skimmed over, but I have no idea where it was in the timeline. I'll drag the slider around trying to find it again, and sometimes I succeed, and sometimes I give up. With the buffered parts highlighted, I'd at least have a better idea where it was located (or better yet, the timeline could highlight the sections I've viewed).
It also sort of acts as a bookmark, to know what you've watched. Sometimes I'll open a long video (1 hour or more), and skip around watching a few parts for 30 seconds here and there. I then realize I want to return to one of the parts I skimmed over, but I have no idea where it was in the timeline. I'll drag the slider around trying to find it again, and sometimes I succeed, and sometimes I give up. With the buffered parts highlighted, I'd at least have a better idea where it was located (or better yet, the timeline could highlight the sections I've viewed).