All the previous posts do well to specifically answer your question as to the dual pixel advantages in focusing. However, there are some possibilities that may or may not actually ever be incorporated into later designs, using dual pixels, which could open up a number of options not now available. Somewhat like Fuji in some of their recent sensor designs, Canon could actually use one pixel in each pair for reading highlights and the other for reading shadows, thereby creating a sort of instant HDR exposure, increasing DR by quite a lot. Other arrangements could also be made to use one of each pair to read color information separately and differently from its mate in the same pair, thereby increasing color depth and accuracy. I'm sure that many things that others could think of, would also be possible , but these are some that sound interesting to me. Whether any of this could or will actually happen is another thing entirely. Maybe Canon already has this in mind, maybe not.