I think being able to edit the subject and take private notes directly attached to the email are really good features.

"take private notes directly attached to the email"

what does this mean?

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).

Fastmail is based in Australia. Some use cases might have issues with that.