BCC is conventionally used to let other people secretly see the contents of an email, where you think for political or business or personal reasons, you need to protect yourself by having a 3rd party witness the exchange.
On 4F we don't need this often because (a) we have nothing to hide and (b) those risks are best avoided.
But we do use bcc: for the following purpose.
In the modern organisation, the individual is obsolete. We need teams, so that we have resilience when people are ill or leave, etc.
So we need to constantly keep our team in the loop about what is going on, so they can pick up our critical work in the event of a mishap. Its also useful to be able to discuss difficult ideas about those projects with them. But there's no need for the receiver to know who is in my team or how I'm managing my team. So our use of bcc encodes two meanings:
(a) The recipient can't see who I'm copying
(b) My team members must NOT join the discussion by replying to any of those emails
In a perfect world, we would have the option to hide the recipient from the bcc'ed members.