dont know if its just a backup mate.
I suspect its a case of your location on the road is determined by a combination of periodic polling of the GPS satellietes, and the cars own calculation of its movement between polling.
So - the satellietes "tell" the car where you are.
The car tracks its own movement using RPM / speed / direction etc.
Next time the satellietes are polled, your location is corrected.
This periodic polling gives you an accurate location and directions, with a reasonable amount of traffic back and forward to the satellietes.
And, should you loose GPS signal, the car will still be able to guide you, based on speed, direction etc.
Once the GPS signal picks up again, the sat nav will correct its position according to the info from the satellietes.