Like while traveling? I always carry spare socks/underwear/t-shirt in my carry-on just in case my luggage goes missing.
Edit: I read it and it seems kinda bullshit. It's including people who carry it with them to the gym or who otherwise have a second set of clothes and need to change later in the day like for a second job. Of course you're going to carry extra underwear if you go to the gym.
I carry a lot of stuff in my truck. Inside and the toolbox. I could probably live a few days just with the stuff in my truck without plannning ahead. Never underwear unless it's in my bag while going somewhere overnight and such.