Should I “update” my Roadmate Magellan GPS

I've heard the arguments of using a GPS device in lieu of a cell phone and vice versa. Yes, the cell phone does require a cell signal to work. However, the phones have a built in GPS (at least mine does). It's my understanding that you only need a cell signal to get going, but once underway and "navigating" the GPS function will continue to work without cell service. I've started navigating with my iPhone in a place that had service, but then drove through an area that absolutely doesn't have service, yet it functioned fine throughout.