Originally Posted by thepolyparrot

I'm not sure, but I think it's also called a fuller foot - you use it for overcasting raw edges with a zig-zag stitch, so that it leaves the edge of the fabric flat, like a serger would do. It won't let the thread pull tight and crinkle the raw edge.

But you can also use it for fulling your satin stitch - it holds the thread up off of the surface of the fabric until several stitches have been secured. It keeps the satin stitch from tunneling and pulling tight and leaves it pretty and smooth on top.