nav is useful when you run into construction zones or closed roads to find alternate routes. even if you dont use it as a nav, you can use the map mode (like me) to see your options immediately. that alone is a big plus. also might be able to wire up the rear view camera and perhaps some obd2 gauge stuff.
i didnt get the nav cause i wanted cloth seats (base) but i have a 5" nuvi wired up in my car.