Re: Google maps does all this already...
The biggest benefit of the in car system is the availability of power, the biggest issue is the usual lack of updates
In-car systems also tend to have access to wheel motion detectors. That in combination with a magnetic compass allows an in-car nav system to continue guidance in, for instance, long tunnels. The TomTom kit like the app in my phone tends to sort of make things up for a while :).
What I really like of the phone version is that it checks the route for traffic jams and tries to reroute me if possible, but I suspect it's not hard to add that to modern car media systems (especially since they may have to dial 112 in the future, so the electronics will already be available).