they complementing through creating maps with garmin platform...so let say if you contribute 2 POIs (Point of Interest), then they will give you the latest updated map file that is usually named gmapsupp.img, but that file is usually to be loaded into NUVIs and also for WinMo's. Garmin for BB is all loaded from the server because we don't have the map preloaded into our BBB.
But I heard rumours stating that, since the BBB has a built in GPS and also has a SDcard slot, they might change the interface to be used like WinMo's and NUVIs without any data charge. Anyways, like I say it's just rumours. All we can do now is hope for the best.