Love all the, why wouldn't I just use my FOB comments. I would LOVE to not have to worry about carrying around my car keys, especially for an active lifestyle. I wear my Apple Watch for almost everything, but I'd rather not have to worry about keeping keys somewhere secure while kayaking, mountain biking, snowboarding, etc. Heck, I dropped the stupid key FOB in the lake once stepping from the boat the dock and had to fish them back out.
You know what doesn't ever fall out of my pockets, my Camelbak, etc.? My watch that is strapped to my arm.
I would even suggest they should expand on the capabilities. Why can't my watch/phone know all my favorite settings and then if I'm in the drivers seat (ultra-wideband is supposed to be location aware) set the car's features to MY default settings no matter what car I'm in. Radio presets, temperature controls, etc. If it's the same kind of car and has memory settings, put the seat and mirrors where it should be, etc.
Imagine if your business has fleet vehicles, and you could just step into any one of them you're authorized for and it is already configured for you. Imagine taking the shuttle to the rental car location and not needing to do anything, your phone or watch is your key, you unlock your car and again, everything is configured to your preferences.
This could also offer the easy ability to "loan" your car to anyone via iMessage and set controls. Time limits, teen driver limitations, etc. could all be set remotely. Maybe you don't need to give a key to a valet at all, maybe you just assign them permission to move your car while you are at X location or for Y amount of time.
Heck, let it pass the information to automatically connect my iPhone to the car's bluetooth so I can use CarPlay without any configuration too.