I used to do that back before scanners and even copy machines! Would write in the journal first, edit, then hand copy to paper and mail. A twelve page letter was a major commitment!

Now I just copy them, including the envelope if I've done anything unusual with it. It's so nice to have both sides of a correspondence! Years later you can look at them and decide if you were Cyrano or a toad.

I also copy the person's letter, and when I reply send that copy to them. A large tuck in?