Once you create a channel mask in one document, it is possible to move that channel mask from document to document. While most channel masks are so specific to a particular document, it wouldn't be practical to move thema channel mask defining a selection of a tree line against a blue skymany channel masks can be used over and over again. For example, a series of channel masks that create unique selection borders around an image. You spent a lot of time creating the borders, and you would like to apply those same border selections to other images. If that's the case, then increase your efficiency by saving them as layer masks and moving them between documents. Not only will it save you a lot of time, but using selections more than once can add a sense of cohesiveness to a design.
Channels palette.