I find the social dynamics of To/CC/BCC really interesting. You only see it in Emails and it doesn't exist in any other communication medium. Most social communication platforms I can think of the top of my head only really support the "To" part of email.
CC and BCC are practical parts of communication but only formalized in a technical manner (beyond their physical carbon counterparts) in email, but we certainly engage in conversations which are of an "FYI" nature all the time.
Source: I am a communications nerd both from a technical and historic perspective.