The whole binding, front and back, in one seam
Binding normally takes two rounds. You sew the strip to the front, fold it over the edge, stitch the back, and hope you caught every inch. Miss a spot and you are unpicking.
This guide changes the order of the work. You feed a flat strip into the guide and it folds the binding around the edge of your quilt right at the needle, front and back at the same time. Both sides are held in place at the exact moment the seam goes in, so the needle catches them together. No pinning, no flipping the quilt, no missed spots on the back. The fold cannot shift, because it is sewn the moment it is made.
One seam around your quilt, and the binding is on.