The BCC header, as the name suggests sends the email to the recipients, but unlike TO and CC the other recipients can’t also see who else has been BCC’ed in.
This is obviously very beneficial when sending the same email to many people and for privacy you shouldn’t be disclosing everyone else’s email address.
I recently received an email from T-mobile UK which did exactly this. A mass email to numerous people, and everybody was in the TO. Details include full names, email addresses and even some phone numbers.