"The preservation of the unsent letter is its arresting feature. Neither the writing nor the sending is remarkable (we often make drafts of letters and discard them), but the gesture of keeping the message when we have no intention of sending it. By saving the letter, we are in some sense 'sending' it after all. We are not relinquishing out idea or dismissing it as foolish or unworthy (as we do when we tear up a latter); on the contrary, we are giving it an extra ote of confidence. We are, in effect, saying that our idea is too precious to be entrusted to the gaze of the actual addressee, who may not grasp its worth, so we 'send' it to his equivalent in fantasy, on whom we can absolutely count for an understanding and appreciative reading" --Janet Malcolm, The Silent Women
I've kept notes for myself, but I've never done what's described in this passage. I actually don't think I've ever really written a serious matter to anyone at all, at least not on paper. I don't have any emails sitting around in craft at the ready either. But I can understand the impulse.