I have a couple of those clamps. They're similar to the right-angle-only picture frame clamps that Adjustable and lots of other folks sell, like the ones in this link:
http://www.ponytools.com/index.php/pony/specialty-tools-and-clamps/angle-clamps, but usable on shapes other than rectangles/squares. Say you wanted to make an eight-sided frame; this clamp would be your friend. I've yet to make an eight-sided frame, but the prospect's always out there...
I've never felt that the saw guides on these clamps were intended for (or at least not successful at) cutting the miter; that's what a miter box is for. But it's not uncommon, on a picture frame, for tiny errors to creep in, resulting in a frame that won't quite close. One classic way to resolve the problem is to clamp up the frame as best you can and cut through the tight corner, thus shaving a bit of wood off each side, until the whole frame will close up tight. These saw guides may have been intended to guide the saw for that solution.
By the way, digressing: if you own any of the right-angle-only clamps, check them for accuracy. I have picked up more of these than I really need, and decided to check them; I wound up rejecting two clamps because they were way out of square.
