You don’t even need a special situation. If I’m at the convini for food, but also happen to need socks/shirt/underwear, and it’s right there, why make a separate trip somewhere else to buy it?
Something as simple as getting caught in rain and needing an emergency change of wet socks meant these convienence stores were lifesaving. Truly something worth emulating in other countries.
The underwear is for all the "Spend the night at xxx" situations you can probably imagine anywhere in the world (pun intended).