I work at home, so I have no commute, and take the bus to weekly volunteer gigs. If I did have a car commute, I'd combine any errand-running with my trip home in the evenings, and leave the car sitting in the driveway all weekend.
I intentionally bought a house in a neighborhood where there are five supermarkets, a Trader Joe's, a produce stand, a post office, two drugstores, a library, a pet supply store, an art supply store, and two hardware stores within easy walking distance. I also live near a major transit hub, so if I need to go further afield, and won't need to carry much home, I can catch a bus. In this town, taking the bus can be much less of a headache than driving, especially if I have to go downtown, so I'm a heavy transit user.
Then there's all the stuff I buy online, taking advantage of coupon codes, specials, subscriptions, and free shipping. Why go out to get nonperishable stuff when I can just have it delivered to my doorstep?
So the car sits unused most of the time. I take it out a couple of times a month for a big Home Depot-Office Max-Costco-Petsmart run. But I keep a list of things I actually need, and shop strictly from the list (unless a truly spectacular deal presents itself), and always make several stops on any given trip.
Efficiency is part of it. But mostly, I just plain hate shopping, and dealing with my fellow shoppers, and driving around town, so getting it all done as quickly as possible, in one shot means I get to kick back and not think about it for a couple of weeks.
The only time I take the car out for a single errand is if I'm taking a cat to the vet on a warm day. On cool days, they're fine waiting in the car while I do other errands on the way home, but I won't leave them in a (potentially) hot car. That happens maybe three times a year, though, so it's not a big deal.