Who knew?
It’s far afield from “civilized discussion”, but . …
Several people are working on, or thinking about, a particular document. The document is written and the discussion is about it. You might just want to highlight stuff in it just as you might have 20 years ago in a pile of bound paper called a “book.” Lots of times you’d want to highlight a section and quote it, but you might just want to light up stuff that you intend to think more about later.