And now Bob and Jack can have a conversation and Sue doesn't get flooded with responses that she's no longer interested in...
I'm managing registrations to dance lessons at our club.
If we need to tell the participants something, I send out mails with From: ourclub, To: ourclub, and Bcc: all participants.
We're not disclosing mail addresses to strangers who just happen to take the same lesson.
I had a fresh-from-uni junior who asked why the hell I put some people in the "to" field and others in "cc" it will be the same result, everyone listed will get the mail. So why bother?
1. Drop copies of things I want to be absolutely sure I have access to, into a source controlled by me without broadcasting that fact. It is super easy with BCC
2. Notify people of a conversation without involving them directly. Usually, this means getting advice, or giving a heads up, should they need to be involved later.
In fact, my mail client is setup to automatically BCC all mail I send.