I use this foot whenever I am adding some detail to purses and purse handles and straps, and when I'm attempting to stitch-in-the-ditch. I place the guide along the edge of the fabric or ditch, change my stitch length (I topstitch with a longer stitch, usually a 3), and keep the fabric lined up with the guide. AMAZING!
The Janome foot is called "The Adjustable Blind Hem Foot," but don't be fooled by the name. What I love about that particular foot is that it is adjustable, so you can "adjust" the distance between the guide and the edge of the fabric.
This foot is a great foot for beginners; the kids in my studio call it "That Cool Foot" because is straightens up their stitches when they topstitch straps for their totes and bags. It works great on machines that don't allow the needle to move.
The video below shows it all!