A couple of things...
Is there any expectation that AirTags would be usable in the Home app?
It might be the ultimate in laziness, but here is one possible AirTag use case: I have a garage door opener that is HomeKit compatible. I was thinking that it might be cool to keep an AirTag in my car and have an automation set up in the Home app such that when my car returns to within, say, 150 ft (geofence) of my garage, the garage door automatically opens.
Another potentially use of keeping an AirTag in your car, is that if your car is ever stolen, you'd know exactly where it is located in the FindMy app.
You can already get the garage door to open automatically via geofencing. Though UWB would obviously be a a lot more accurate in knowing when you're in front of the garage door, and not the street directly behind your home.