I always thought it was about getting the parents outside for some exercise and social contact with non-babies.
Being ‘strolled’ soothes a lot of babies into sleeping, as does riding in the car… I know a lot of parents who have taken their baby out for a stroll or an aimless car trip just so it will nap.
I have lived in 6 different states, including in the South and in Southern California. In every place I have lived there have been plenty of people who walk their kids and walk their dogs. Maybe I am being whooshed here.
Yeah, I was surprised and somewhat dismayed to hear that large areas of the country don’t walk their dogs. Like feeding them a diet more nutritious than table scraps, walking your dog is a major part of the responsibility of dog ownership. I would like to think that Sun Belt dog owners engage in other forms of exercise with their dogs, like Frisbee or agility training.
Some kids are really soothed or fall asleep in a stroller. Most kids really like watching the world go by. Me, i really enjoyed the exercise and taking the kids around. When we had twins, me taking the twins out for an hour or two was greatly appreciated. I got good exercise and explored a 5 km radius around my home in Shanghai in wonderful detail. And modern strollers have a built in beer holder for Dad. Pleasant summer evenings out for a stroll with my twins around the 'hood with a cold one or two - life doesn’t provide many better simple pleasures. 
While some strolling might be for baby’s benefit, generally babies are put in strollers because the parents can’t leave them alone at home. So babies go where the parent goes, for whatever the reason the parent is leaving the home.
In my neighborhood it’s so the mom or nannies can get out of the house for a spell.
If my friends and I happen to be drinking beer on the driveway, they’ll usually stop by for a brew too.
Dads are the dog walkers, moms and nannies are the stroller pushers.
Walking in a stroller was the only reliable way I could get my baby to sleep. Riding in a car wouldn’t do it. Rocking or swinging were hit and miss. But take a walk in a stroller and he was out within minutes. That’s the only reason I ever did it.
Besides that, there are only so many things you can do to entertain an infant. Sometimes you need to get the heck out of the house.
I imagine it may depend more on the specific neighborhood than the geographic location. I used to live in South Miami, less than 200 yards from Coral Gables. My neighborhood was not built for walking (the immense majority of the neighborhoods I saw around there weren’t; I spent 3 years in South Miami and 1 in North Miami, this last year going to Miami Beach to work); I never saw someone taking a kid for a stroll, or a dog out on a leash, in the neighborhood. I did see people loading the stroller in the car, I did see dogs in yards. In Coral Gables, which had wide, well-kept sidewalks, I did see both dogs and kids being taken out for strolls. Again, this was two side-by-side locations.
Parents who never walk are unlikely to start walking for the kid, I imagine.
Geez people don’t you know babies are super powerful? They actually command the parent or even a nanny to push them around. Why? They don’t like it, it’s all about the power. They get off on it. It’s like a huge ego trip to them. They are all like “I am baby, do my bidding? Feed me, wash me, walk me.”
Of course the superpowers wear off as the baby grows, so the baby forgets he/she had them
When I first opened this thread yesterday afternoon, my husband walked in the door, carrying our almost 4-month old kid, who he’d just taken out for a stroll. The kid likes it. The husband likes it. They bond. They don’t use a stroller to stroll, but one of those wrap things.
Seriously, tiny kids seem to enjoy all sorts of things you wouldn’t think they’d care about… we read books to ours before bed, knowing full well he doesn’t understand a thing in any of them, because he gets all quiet and stares at the pictures when we do, even if he had previously been fussing and carrying on. Another even more sure-fire baby-mood-improver is to take him in the bathroom and lay him down on the changing pad, even if he has a perfectly clean diaper and all he does is lie there. He chuckles to himself and turns his head this way and that, grinning at the blue-green tile on the walls and the fluorescent lights. He just likes the bathroom.
ETA: Proof.
Kids are weird. Also, **Markxxx **is correct.
My son liked it as an infant and still likes it as a toddler. If we do it around his nap time, he will fall asleep quickly, and we gt some exercise.
It really is that simple.
Boredom.
Babies like it.
Where I live many babies and young children are strolled (more often jogged or power-walked with their moms in running gear) but I know for a fact there are tons of dogs in this neighborhood - which is lovely and suburban with few busy streets and nice clean sidewalks - and few of them get walked. I know this because I’m out with mine most mornings, afternoons, and early evenings, and it’s the same few we pass by every day while all the rest bark at us from the front window or the yard. Older children do not spend much time playing outside here either, which makes me sad.
NOM. Your son is ridiculously adorable.
Thank you, rhubarbarin, for that completely unsolicited compliment which I totally was not fishing for!
Your check is in the mail.
He was looking a little robotic there before he smiled. 
another “best” way is putting the baby on top of the dryer.*
The clothes get dry, the baby gets rocked to sleep, and you get some peace and quiet
*(sitting in her car seat, ya know . 
)