Just a guess: maybe they treat email threads as a document. You can then add comments to it (i.e just notes for yourself and not for others)

I get it now, thanks y'all.

Imagine using a draft reply in Gmail as a notepad on a thread. It's that plus a few other things, but you don't have to worry about accidentally hitting send at some point, it's properly designated as a note (how the interfaces displays it, I don't know).