You click Reply. You type some stuff. You click Save, to complete your draft another time. You close your unsent message. Outlook labels the original message -- the one you haven't yet sent your reply to -- as, "You replied on [date and time you clicked Save]." But you haven't actually clicked Send yet!
You go into your Drafts folder. You delete the draft. Outlook still labels your message as, "You replied on [date and time you clicked Save]," even though you not only never replied, but you have even deleted your unset response!
I can only imagine the misunderstandings this causes. "You never replied to my message!" "But I did reply! It says I replied!!"