Before email communication took over law practice, lawyers sent letters. Lots of letters. And they often would send their clients carbon copies (CCs) or blind carbon copies (BCCs), even of letters sent to the court or opposing counsel. This system worked well because the clients’ responses came back directly to the lawyer—and no one else.
This system also tends to work well for email communication, where it has been adopted into daily practice for thousands of attorneys. But it breaks down when you get an email from opposing counsel and CC your client on the reply, or when you need to contact the judge and BCC your client to keep them informed. Bad idea.