The term carbon copy refers to copying a document being written by putting a piece of carbon paper in between two sheets of paper, before typing/writing on the top piece of paper.
When writing an e-mail the CC field is used for sending copies to other recipients. The BCC field is used for sending copies to other recipients without informing the initial recipient(s) about it.
An email-address is a piece of contact information. Similar to a phone number.
I don’t share my phone number with many people. Having a phone number entails responsibility. I assume that people who have my phone number either (a) knows common etiquette concerning phone calls or (b) have been informed about when I accept calls, i.e. my office hours. To share my phone number without my consent is to abuse my trust.
Another form of contact information is a physical address. Don’t get me wrong, I enjoy having visitors. People with whom I’ve shared my physical address, are always welcome to pop by for a cup of coffee. I’m not going to turn down a stranger either, if they’re in dire need of a cup of coffee.
I like sending physical mail as well. I often get a lot of letters, and I often dedicate quite some time to going through my mail. If my address was shared, without my consent, with someone who would send me spam, advertisements and letters that did not concern me, I’d regard that as a betrayal of my trust.
Apart from protecting privacy, BCC has other advantages:
If the email is forwarded, BCC addresses will not be forwarded as well.
If the received mailbox is compromised, or the mail is leaked, no superfluous emails will have leaked.
BCC has the disadvantage, that its not being honest about the recipients of said email. This is why we have been using mailing lists since the end of the eighties. A mailing list consists of an administrator, subscribers, and an email address. Whenever an email is sent to the corresponding email-address, every subscriber on the list receives a copy. The administrator is responsible for managing the subscribers. This task has the past several years been automated, to allow subscribers to join and leave lists as they please.