Since I don't do it this way, I'm assuming that like in the freezer paper method its to stabilize the pattern, then iron the edges back around and iron shape down onto the fabric (that way you wouldn't need pins) and then sew a decorative stitching around. The only thing with using a stabilizer is it does make your applique stiffer on the quilt top, unless you choose the water soluable kind (which I don't have any experience with).