an applied double binding is 2 thickness of fabric on both the front and back of the quilt.
when you fold the backing over the front you don't have double thickness on both the front and the back.
most people say that the binding area takes alot of wear and tear and if you don't use double thickness the binding will wear out.
i don't know if this is true since 99% of my work is art quilts that hang on walls.
i always do double binding on my quilts.