Best use case is if you've gone for a walk or have more cars than people and then what, your car in your driveway becomes a glorified 5 figure oversized mailbox?
Call me paranoid, but I feel like the real reason is this:
>On the day of delivery, customers can check if their car is parked in-range of the delivery location in the Amazon Key app.
That implies to me they'll have access to your cars location, which is information they can use for targeted ads.