Something else that Google Maps can do is send anonymous info about speed and location to build up a ‘live traffic map’. This helps give more accurate predictions for traffic and journey time. It doesn’t just apply to driving, either. You can also use it when you’re cycling or walking, so long as you choose that option on Maps.
I think that having a sat nav in the car is handy for times when your phone battery is low or you can’t get a signal. Overall though, Google Maps works just as well as a sat nav and has the added bonus of automatic updates.