I would like to give many addresses comma-separated in the System configuration key MAIL_SEND_BCC_TO_ADDRESS.
My usecase is that I (as an implementor) use this function for debugging while my customer is using it. We both can better communicate if we get the same mails. Especially while creating a process for mass invoice mailings this was very helpful.
Another usecase can be a second bcc address for a kind of archive that collects all mails for future reference.