Spray starch adds body to fabric. It cuts down on fraying, and when used with applique, it stops frayed edges and makes needle turning easier. If applied before rotary cutting, the fabric does not shift, and your 1/4" seams are straighter when you sew. Blocks square up easier and there are less waves when sashing is applied. When machine quilting, the sprayed backing glides smoothly on your work surface and there is less puckering on the back. Bias cuts have less stretch. Pencil marks also wash out easier. Sizing spray is softer than spray starch and works just the same.