You were looking for an example of an app that requires Bluetooth. I recently installed Torque. It is an application that requires a Bluetooth OBD-II interface (about $30 online) and turns your phone into an automotive diagnostic tool. It also uses GPS and the accelerometer in your phone to give you detailed movement information about your car. (All cars made for the US market since about 1996 have been required to have an OBD-II interface; many states use them for their diagnostic testing.) It will also give you any error codes that your engine control module is reporting, e.g., for a "check engine light". It may save you from having to pay a diagnostic fee to a shop just to find out what is wrong.
In any case, this is an example of an application that will require Bluetooth. In my opinion, it is an excellent application and is free (ad supported) or a few dollars for a non-ad upgrade.