During personal correspondance by physical mail should one make copies of out bound letters?

This was never my practice. There are two main reasons that I think this is an important step. Number one is that if the recipient responds questionably, a backup is available to reference back to exactly what you wrote, and see that they are right or wrong. Number two is to insure that you receive a response on all the topics you covered if a response was requested…or expected. Someone’s omission of a subject in a response may have great meaning.

Is a written letter so important that its contents are all carefully memorized by some?