When my wife and I had children, our respective parents, two very different families (one progressive professional urbanites, the other conservative rural farmers) were both unusually insistent that our babies "get dirty." I can still picture them sitting on the ground in the garden in their diapers, grabbing handfuls of dirt and sticking them into their mouths (they learned quick, though). My mother, a paragon of modern conveniences, was insistent we hand wash our dishes. And you'd better damned well breast feed unless you've got a really good reason not to.
I was raised to be independent and think for myself, and my internal voice was saying, "What's with all this hippie B.S.?"
When asked about it, my parents told us that these ideas were handed down from their parents (coincidentally, farmers and business-people) after generations of observing that kids who grew up "too-clean" got sick more often. Literally, wives-tales. But as an adult, exposed to other families through school and playmates, I saw this for myself. Probably confirmation bias, except for now it's got science behind it.
Funny how some old knowledge becomes new knowledge again.