It confirms to Mark that you received his email and are responding to Suzanne but leaves him out of the rest of the conversation when Suzanne replies.

You say thanks to Mark, but save him countless minutes of pressing Delete on the future CC's back and forth. Busy people get already too much e-mails and it is often a pain to get out from those discussions.

Most people don't make a good use of BCC.

Another instance where it is useful is when asking a question at a mail list but asking that anyone who answers only addresses the you not the whole email list.

Just send the email with the list in BCC, and that's gonna be automatic.

And then face the rage of a thousand suns when you break email filtering rules ;)

(personally I have rules that catch this sort of behavior so it doesn't bother me, but I've seen it happen).

That's a pretty rude and selfish way to approach a mailing list.

I wouldn't be surprised if such a question goes on to be ignored.

There are uses:

    From: me
    To: me
    Bcc: foo-enthusiasts
    Subject: [survey] what's your favorite version?

    What's your favorite version of foo?  Write me off-list
    and I'll post back to summarize.

From RFC 1855, Netiquette Guidelines, Section 3.1.2, Mailing List Guidelines:

> If you ask a question, be sure to post a summary. When doing so, truly summarize rather than send a cumulation of the messages you receive.

http://www.apps.ietf.org/rfc/rfc1855.html#page-10

Imagine the maillist of all your coworkers and you want to let them know you are selling a few items. It's not rude to force responses to be only to you. This reduces the amount of email everyone gets.

No, that's what Reply-To is for.

Except gmail doesn't support reply-to in a per message basis.