Reasonably frequently, I find myself replying to mails I have sent to others. For example, I might come across additional relevant information after sending the original, and want some context for a new message. A neat way to do this is to find the original in the Sent folder, reply to it, trim the quoted text if applicable, and append the new information.
Now, I can't think of any other reason to reply to one's own Sent mail. In particular, I can conceive of no universe in which I would want the recipient of the new reply to be me. And yet this is Apple Mail's default behaviour. (Did I say “default behaviour”? That makes it sound like there's a preference to select sane behaviour. If there is, I cannot find it.) If you reply to a mail which has your name in the ‘From’ field, you'll be the recipient listed in the ‘To’ field. This happens with Reply (Cmd-R), as well as Reply All (Shift-Cmd-R). The only redeeming feature is that the original recipients are preserved in the ‘Cc’ field. They can be manually cut and pasted into ‘To’. A real mailer would not make me do this.