Bev: I'm no expert but I think that's because when you quilt the body of your quilt it can "shrink up" your backing and batting and you wouldn't want to have areas that aren't backed. I agree tho that sometimes it seems like a waste of fabric but I'd rather have that extra to cut off than have to add more fabric to an already quilted quilt back.
I save all the fabric that's left because I can cut them easily into 2-3 inch strips for future use.
Maybe someone else here can give you more info-this is just my humble 2 cents worth.
Oh, the extra hanging out the ends also makes sure that your binding will have batting in it too I think.