It's sort of a "general purpose" foot - the idea that you could leave it on your machine instead of changing between feet when you wanted to do different things. It never caught on, one reason why there are so few of them out there, and that makes it valuable because there are crazy people (like me) that just have to have ALL of the Singer feet . . . in no way is it "worth" that for usability, and most people agree that the individual feet work better.
They come up with like 17 different functions, but some are basically the same thing, just using different types of trim. The April1930 link shows many of the functions, and at the bottom has a couple of pictures of the instruction manual showing what Singer says it can do. Mostly variations of attaching trim, gathering, and small tucks. Nothing really fun like the embroidery attachments!