I read the description, I think it kills the viewing angle on your cell phone while acting like a screen protector. In this particular case, the viewing angle is cut down to 45 degrees - I presume that anyone looking over your shoulder (or trying to) would see the image instead. That's why'd you'd use it, too, to stop that sort of stuff from happening.
I suppose you could use it to spread an aura of moe around yourself whenever you take out your phone, too. Either or.
