Originally Posted by ckcowl

The reason for extra batting/ backing is due to the fabrics ( drawing up) with the quilt process, much like when doing applique you make your blocks larger, stitch the appliques, then trim to the correct size. When quilting the process causes the fabrics to draw up, become slightly distorted. Having the extra makes it possible to trim/ square up after quilting, before binding. Also, you start quilting in the center, working your way out to the edges so any distortion is eased out along the edges. If your edges are already finished there is no where for any fullness to go, which could cause unsightly wrinkles, creases, pleats in your quilt.