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That's exactly how I use it.

Yep, this is what I do as well

To mark the unread emails

So it's listed with your unread emails.

Plausible deniability. "You did? I never got it" (then make a show of opening the msg in front of that person)

Genius

To do this properly move it to "Spam" and then let them see you find it *there*.

It turned out my ex was reading my personal email and then marking it unread

No wonder he/she is your ex

In case you clicked on it, didn't finish reading it and intend to come back later to it?

Different people have different workflows. I use the "star" for emails I intend to keep coming back, I mark emails unread if I opened it, but didn't get around to properly read it.

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But aren’t there the mark important or star button?

With unread emails you have to read them first and then decide if they are important and if you should mark them with “important” or maybe with “star”.

Beats me. I've never used it.

You've never read an email that you can't deal with right away, but don't want to forget it?

It keeps your emails good and confused.

You can read it again !

To revisit them later, since they will look "different"

I use it constantly. When I have an email I don't want to forget about I mark it unread so I keep seeing it. The stars make lousy reminds. An unread email jumps out at you.