A large part of the beauty and potential of alpha channels is how sophisticated they can be. When working in photomontage we most often desire a mask that eliminates a significant portion of an image element (typically a background) so that it can be composited into a base image. This helps create the illusion of unity and believability of the final composition. Yet, there are times when a mask should have translucency in at least some parts of it in order to  make the results look real.