Android only: Why would anyone need a turn-by-turn navigation app for Android, besides Google's own built-in offering? Travellers and non-US users, for one, but also those intrigued by the OpenStreetMap project and its alternative local data.
International coverage of streets is a handy thing for those travelling abroad, as well as anyone looking for a turn-by-turn solution while outside the US. But OpenStreetMap, with its user-contributed notable places, traffic/police warnings, and sometimes improved understandings of local traffic patterns, are worth checking out as well. MapQuest's GPS offering is also an entirely free app for its lifetime, so you've got nothing to lose.