I definitely wouldn't wear my gear or sleeping tanks for working at the office (fair game for wfh though), but they could be for casual. My normal work things I'll often wear on weekends too. Unless I'm doing something active I don't dress down much on weekends, just 1 more casual piece.
I think of my things along Imogen's "refinement levels"- low is gear and sleepwear, mid is daily work and weekend, high is formal work and festive. I may cross pieces mid and high- like wear my tall heeled boots to a normal office day, but never low and high.
Anyway....the point was that black pull on tops are a category that must be available for every occasion for me. So essential in my gear capsule, essential for sleep, etc...