- A call or text at an inopportune time doesn't inhibit navigation.
- The dedicated UI is simpler and doesn't change; my GPS is about 8 years old and has stayed the same the whole time. Every Maps app I've had has changed a dozen damn times since then; I value the stable interface more than the bullshit bells and whistles. It needs to show me directions and be otherwise unobtrusive; a phone doesn't do that.
- Sometimes I turn my phone off to eliminate distractions
- Sometimes I'll hand my phone to my kid to play a game
A phone does a thousand things, but none of them as well as a dedicated device can do 1 or 2 items from the list.