After looking at my manual again, I realized that mine came with the machine too (Sears exclusive Singer, which is actually a Kenmore). I use it ALL the time in making diapers. I can so easily get a nice zig zag on the edge of fabric with this foot. With a regular zig zag, I'd have stitches that weren't quite at the edge. But maybe that's just because I'm not a great sewer.
