In any situation where voice commands are better than manual input, but where noise/silence is a concern, this could be useful.
If your hands are dirty and you don't want to touch the screen, but maybe it's important to be quiet, then the gesture could be nice.
If you are cooking and want to see the next page on the recipe,
If you are eating a greasy pizza and reading Toms and don't want to tissue your fingers every three seconds.
If you are the kind of person who cleans his screen every 2.5 seconds to remove any fingerprints.