I can't even believe anyone offers dedicated proprietary nav anymore in cars with Android Auto/Apple Car Play.
For BMW, Audi and other brands that comes with HUD (Head-Up Display), right now only the proprietary nav is compatible to project detailed route info to the HUD. This case it makes sense. For VW, you can put the map on your digital dash if you got one, it can help familiarize with the road or can be handy when you are in a new place.
Also, for now, Android Auto and CarPlay can only use the phone's GPS. In downtown of the cities the signal can be very weak and the positioning is not accurate enough. The phone's map cannot tell which direction you are heading if you're stopped. This issue can be very annoying navigating around the city's roads.
For roadtrips and adventures, sometimes you will go through places without any cellular service. Car's GPS is a lot more reliable if you update the map frequent enough. Phone can keep navigation going but search for new POIs are crippled.
Other than these cases, I will always prefer Google Maps.