I cannot answer from a personal standpoint since up to now I am only a "quilt-top quilter" I'm going to get there soon but I can tell you what my mother said is the reason to do it that way. You machine sew the binding on the front and can make a nice even straight line of it because you have the whole strip of binding to make sure you have even straight coverage. You hand sew the binding to the back in case your binding width ends up waving between too short and too long. You can ease the difference in a little nip tuck at a time. Hope this makes sense.