Carbon copy

Carbon copy used to be a mechanism to copy a letter while it was being written.

It served a need: If you are about to write a multipage letter to someone, you will want a copy to keep for yourself.

Sometimes to reference when the reply comes back. But I believe mostly: To re-read and archive what past-self had to say. To reflect and learn about yourself.

This need is still being satisfied by most writing to this day.