> The reason why was not is because, prior to computer, there was not a problem of "sharing" the key

Lol. There is a new dude sitting in a castle one sea over. You want to communicate with them. Let’s say you want to agree to attack a third neighbour at the same time. If your message gets intercepted you will be in trouble.

How do you share your secret keys with them? Mind you, you don’t really trust any of your man, and the sea route is treacherous so you don’t fancy risking going in person.

Sharing keys is a problem old as time.