Because I got stuff I want to keep in my car. When I go shopping I sometimes hit more than one store and don't want to carry everything from Costco with me into Home Depo.
Also dogs.
Also dogs.
A manual lockout would be a smart thing here. There's all kinds of solutions to the "computers are dumb" problem.
I also think it's slightly less error prone, as there's no user input required, and since the stuff gets removed from the car, it's much less likely that anything will end up being sent to another user. At worst, they forget to put your stuff back automatically, and it's relatively easy for them to send another car, or even send your stuff to your house. Indeed, that might even be a service people are interested in if they're doing a lot of shopping, or they take public transport for part of their journey.