Here are a couple of possibilities. Working aged person sends their car to their elderly parent's place so that they can use the vehicle to do their groceries. Families with kids in various activities can get the car to deliver and pickup all the kids at the appropriate times without needing multiple vehicles if the parents need to accompany some of the children. Car pooling becomes more acceptable because you can sleep during the detours to pick people up.

In reality, I don't think it is useful to try to enumerate these small immediate changes that are distinct from the availability of taxis. The long term cultural shift of having autonomous vehicles may lead people to fundamentally share vehicles in a different way. This may lead to a situation where fewer vehicles are driving more miles.