Still don't see it

So the New York Times had an article this morning about shape-note singing, and talk a little bit about how the shape-note singing method works. What I don't understand, though, is that the shape-notes are placed on the standard staff, which does exactly the same job as using different shapes. It'd make some sense if the system were used in lieu of a standard staff, but just adding it on top makes absolutely no sense. If anybody out there can explain it to me, feel free.