Well a 4g connection is faster than most domestic broadband... and even so, if you were on your drive then you could connect your phone to your home wifi... mind you, the slowest component will be the bluetooth connection between your phone and the car.
Just makes it easier than downloading, copying to a card then updating the system from the card.... or for people who increasingly don't have a computer, but instead an ipad or just phone.
Would have made more sense for the car to have a wifi client and or its own 4g modem.... the former probably does exist in the unit (but not active) and the latter was how the original carnet stuff worked with MIB1 (but not in the UK!)