It's more accurate, nothing to do with an extended range. BT is. just relying on the strength on the signal, which depends on your distance from the antenna.
UWB can precisely locate you, all around the car, within a few inches precision. It will know if you are walking toward the driver or passenger door for instance.
That way it could fix the leg squeeze when you and your wife are both entering the car with your phone keys and the wrong profile get selected.
UWB could lock the car faster. Currently the car locks when it loses the BT signal or if it's very low + a couple seconds delay. I often find myself on the other side of the street at more than 10-15m when I hear the lock sound. I think that's too late. UWB can do that just after you've taken a couple steps, which would be an improvement.
Re unlocking, I've had several occasions where the car didn't recognise the phone, and I had to take it from my pocket (and sometimes unlock it) for the car to unlock as well. UWB could fix that.
It also consumes less battery than BT-LE.
And maybe, finally, it will allow Summon to be actually used up to 6m according to UNECE rules, instead of having to practically stand on the bonnet for it to work at the moment.
But of course, this would mean finally getting Summon for non-USS cars, as there isn't one with UWB+USS at the moment..