I guess there is really nothing majorly functional about this app. What I mean is that it doesn't necessarily help you do important things.
But, it's a great party trick. Just point your phone toward the sky and you can see where all the constellations are placed in the sky. Forget the days of trying to figure out if that star is part of the Big Dipper. This app shows you exactly where to find those constellations.
There is a setting to change it to night time, so that everything is in black and red. This keeps from distracting your eyes with light.
So, it is sort of like carrying a little planetarium in your pocket.