As a young artist I believed that masking agents (tape, film, drawing gum or liquid masking fluid) created stiff, contrived watercolors. As the years pass, so has my opinion. I now think that these materials can be a great aid. For example, try painting a smooth, gradated sky around some light-toned, lacey branches. Well, you get the picture: it isn't easy unless you mask the lights. Also there are numerous textures that you can create by applying masking agents. Go to my website for my Winter 2010/11 newsletter . It's a short lesson in masking with a demo painting.
"New Snow, Grizzley Creek" is one example of frisket at work.