With the tech giant's idea, a smartphone would download a map of the wireless network signal strength for nearby regions, and it would then feed that info to the vehicle's navigation system. When plotting a course, Apple suggests a few ways that the nav unit could incorporate this data. For example, a driver might get multiple routing options, including the one with the best cellular signal. Also, an algorithm could take this factor into account when finding the way. In some cases according to the patent, "the system can select a longer route having better signal strength over a shorter route having worse signal strength."
Assuming maintaining a cellular signal is of vital importance to a driver, this system could be quite useful. As with any patent, though, there's no guarantee that Apple actually incorporating the tech into smartphones. The filing for this was first done in August 2012, but it wasn't published until May 2015.