It’s usually for the kid’s safety. Little kids especially run off sometimes; and while it’d be nice to be able to be a continuously attentive parent, sometimes you need to get shit done out and about while they’re with you. Sometimes they’re fine with just being carried or sitting in a shopping cart, but if not, a leash & harness (usually just integrated into a backpack) lets them wander safely while you take care of what you’re there to do.

I don’t think I’ve ever seen a kid older than 4 or so in one.