Sometimes when sending an email, a network error or other error can occur. Other times, Thunderbird takes a very long time when sending a message and can freeze or crash, making it necessary to use Task Manager (or Ctrl+Alt+Delete on Windows) to close Thunderbird. After any of these occurrences, it is often unclear whether or not your message was sent, so it would be nice having a quick way to resend the same "sent" message (or message you attempted to send).
As others previously mentioned, Edit as New from the Message menu works too. Ctrl+E on Windows is currently the fastest way. The command may be different on Apple Mac computers.