Phases is one of those apps that has a lot of information you wouldn't really think of or need -- until you actually have an app available to give it to you. Basically, Phases tells you everything you need to know about the moon, from when it wanes and waxes, rises and sets, and when and where it appears in the sky. Sure, it might not be the most pressing information to have, but it's kind of fun to check and see if it really is a full moon out, or find out just when the moon will be up in the evening.
And you don't even need an Internet connection -- the app has a database of over 20,000 cities, so even if you can't hook up to the 'net, you can still get information from the app. It's just 99 cents in the store right now, so if the clean design and surprising amount of data available in this one appeals to you, definitely give it a download.