Starch, sizing, Mary Ellen's Best Press - all are used on the fabric before it is cut to add body. This makes the fabric behave better while cutting and sewing. It lies flatter, doesn't distort as easily, and the corners stay more corner-like. Starch can also be used on a backing to keep it flatter during quilting. You can also starch a block while squaring it to make it hold its shape better. The starch washes out when you wash the quilt.