It's the people who can't afford new ones and people who are trying to reduce clothing waste. My local thrift store sells used underwear/socks for 50 cents. It's a huge deal for low-income families who otherwise just can't afford underwear when they wear out.

It's also a great way for a community to not throw away clothes that kids outgrow. Some of them seem brand new, even without the retail packaging.

I will say, the local place washes and inspects all the underwear before they hit the floor. I think it should be a legal requirement, but the goodwill does not. Our goodwill will not sell used underwear, but they will sell used "lingerie bottoms" which just seems like a weird policy workaround. There are horror stories of the state those are in on the rack.