They should allow you to loot underwear and clothes along with armor off of fallen enemies.
You actually can with NPCs who actually do wear underwear, but only if you knock them out prior to doing so. That's how I took Minthara's underwear, gotta knock her out first
I don't think you must knock them out. I had taken Shadowheart's clothes and underwear off at camp, and after the long rest wanted to put them back on. So I clicked on the clothes in her tab, but they didn't go to her but to my PC because she had focus at that time.
So all the party members can switch their clothes just like that.
