Sometimes it's hard to understand what problem something solves because you may not be in the target market/demographic.

Pinterest? I have a large number of friends that have used Pinterest to help come up with ideas for apartments, weddings, etc., Previously the only place they could look for inspiration was through online searches or magazines.

Skype? How did you call overseas <edit>for free</edit> prior to Skype? I was able to do it with my Ham Radio license, but for that I had to take a test and buy equipment -- with Skype you just had to install an application and you could run with it.

As for the other game categories, I agree with John, entertainment has a huge demand with a market that seems to be ready for the next exciting game at every corner, Angry Birds and Little Wings filled that demand.