gif example, made in GIMP:
Spoiler:This could be useful for any number of things, including pseudo-fog effects with soft boundaries. Even better would be if shaders additional masks were available, such as height (gradient from lowest to highest point), floor, ceiling, walls.
Another example, using a fade to simulate how a heightmask and some "cloud" noise could be used to make a toxic "floor fog"
Spoiler:I imagine masks like this would be useful for things like water effects too.