Everybody has a cell phone. And since everybody has a cell phone, people will drive right past you on a lonely road when your car has broken down, and figure you're just waiting for a lift. And they won't know that your cell phone battery is dead and you have no way to charge it. It's a long walk to the nearest payphone these days. Why make that journey carrying a dead cellphone and a pocket full of quarters when you could just keep a mobcharger in your glove compartment?
The FAQ page lays it all out. You'll need a different model mobcharger for each configuration of phone charger, but there aren't that many permutations, really. It might save you from inconvenience or embarrassment someday. On a cold lonely road some night, it might save your skin.