The mobile app craze, which was started by Apple's iPhone and has been perpetuated by any smartphone worth its plastic, shows no signs of slowing down, according to Gartner. The market researcher says mobile app revenue, which equaled $US4.2 billion last year, will hit $29.5 billion in 2013. For consumers, that's a good thing, because mobile apps rule. There's been some talk of Web apps eventually killing off the downloadable app market, and that may hold true in the long, long run. But, right now, mobile apps rule. Here are five reasons why.