Generally speaking, a difference mask is useful when you want to make an adjustment layer but the adjustment isn't supported by that function. For example - shadow and highlights. Once you run it, that's it, you can't ever go back and tweak it. So . . . create a new layer (paste visible), run shadow and highlights but only for either shadows or highlights, not both, and then create the difference mask as shown in the book. Using that as a mask, run a curves adjustment layer, adjusting for either shadows or highlights as per your original S&H run. Do this a second time for the opposite, and now you have two curves adjustment layers that effect only those places where the original S&H would have effected your image. Now you can go back and adjust it at any step in the future.