Making the iPhone more sticky seems like a solid play. Use the phone in place of a key. Keep maintenance records on the phone. Tap a few buttons on the app and give the dealer a virtual key to the car so you can get it serviced. States are already putting IDs in to apps and the wallet. etc.. Maybe use the phone for various road passes and such, that seems like an easy and natural extension.
Around the time the self-driving craze sort of took off, the noise in the echo chamber was that Apple was terribly far behind in 2 main categories: services and AI/ML. Now, they're charging ahead with services and they've got custom ML hardware on every single device they sell. A car seems like too big of a project with too much hype to use as a forcing function for all that stuff.
Seems like there are some strong health and safety plays as well. If you have a watch on, it already can do fall detection. Car crash detection is a logical next step, if all the passengers had watches on, they could start sending real-time telemetry to the paramedics, maybe encourage them to prioritize a passenger that was in greater distress. With the cameras and such, there is absolutely enough processing power in your iPhone to look for drivers falling asleep and with some other data they could probably make a pretty good guess if you're intoxicated.
My concern was that Apple would make a car and it would be a McLaren or something, it would be coveted, look amazing, and be just about completely unattainable. Now if I could buy a Toyota or a Hyundai and just plug my phone in and it became the brain of the car? I'd talk myself in to taking a new car for a test drive to try it.