I can think of some possible causes of this - cloth diapers and you wash them yourself instead of using a service, multiple hard-playing kids in various sports who practice several days a week and constantly come home with dirty, smelly uniforms that need to be ready for tomorrow, etc. - but really don't think that suffices for all of them.
I posed this question to my mother once, and she thinks they're families who use towels once and then wash them. Which is, frankly, alien to me. :) I acknowledge that my family may be on the far end of the bell curve, but living in the middle of the bush in Africa for two years, having to wash your own clothes and towels by hand in the bathtub* knocks any desire whatsoever to not wear your clothes until you can't stand them anymore before washing right out of your head. Back in the States with a washer and dryer, yeah, we wash them before they hit the point where they can stand up on their own, but I still use towels for several days before washing and tend to wear most of my clothing (not socks or underwear, I hasten to add!) at least twice each before washing, unless I've sweated in them, dropped food on them, been near someone smoking in them, or they are otherwise obviously dirty.
I also practice laundry Darwinism and throw everything in together and wash on warm. It's survival of the fittest: if it can't survive that, then I don't want to wear it. (The three items of clothing which have dyes that run excepted. I wash them at the same time in cold. I also wear mostly knits, so I don't have to iron or hang them.) Toby, OTOH, sorts his laundry and washes it all on different temperatures because that's the way his mother did it, as a result of which he tends to do 2 or 3 loads a week, while I tend to do 1. :) At any rate, when I was a kid and we were back in the States and had a washer and dryer, Mom still never did more than a few loads a week, let alone multiple loads every day!
So. Anyway. Back to the original question: If you have a kid or kids and wash multiple loads a day or are familiar with someone who does, why?
* We had a laundry tub on the back porch, but once an old Africa hand gave my mom the tip of washing them in the bathtub, where you could get in and stomp around to agitate the clothing instead of rubbing them against each other and squeezing them out with your hands, we never looked back. Especially as that meant she could enlist my help.**
** FYI, we draped our underwear over the acacia tree out back to dry. You just knocked any ants off before bringing them back into the house.***
*** Acacia have a nifty symbiotic relationship with a species of ant. Google it. You'll be amazed.
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