@zamorph
As far as I know if you have "phone" built into the car itself(with all radio and other parts), so in order for this inbuilt "phone" to be able to register on cellular network and make calls you use rSAP thus sharing the SIM remotely.
When you only initiating and interacting with the calls from you car interface, but actual call is performed by the handset itself you only need PBAP to access the contacts stored in handset from car UI.