It's important for any developer to know what their potential users are looking for in their software, and according to this visualization concerning Android and iOS users, it's pretty straight-forward: They mostly want games. And if not games, then Facebook. Take a look:
After that comes a collection of browsers (I wonder how often they're being used for games and Facebook, though), and according to TheNextWeb, the data was collected from mobile users through the first three months of 2014.
Particularly interesting is productivity at 4%. That really communicates what people do and don't want from their mobile devices, right?