I can pre-heat or pre-cool my Tesla from my phone, unlock it, check battery state, start/stop charging, see location etc. Doing this from my wrist would be nice. Within a few years every car will be connected to the internet and have similar remote functions. I think controlling automobile functions will be a popular AppleWatch functionality.
The Tesla 85D just released (and the Model X coming out later this year) has all the sensors for autopilot, which will be enabled in software later this year. On non-public roads, you'll be able to tell the car to drive out of your garage to meet you. How cool would it be to tap on your wrist and have your car come to you and open the door. BatMobile anyone?