Are they joking or is it really so common to spend nights at the office that convenience stores actually sell underwear?!
The underwear is for all the "Spend the night at xxx" situations you can probably imagine anywhere in the world (pun intended).
Ran out of socks a few times when I was there. Top up was a 10s walk from the hotel. :)
It's worth noting that in Japan, there are also vending machines that sell used women's underwear.
While the plural is technically true, it is a little disingenuous. There are very few, probably not enough to overflow the fingers of both hands (or even, maybe one hand) in the entirety of Japan.