There may be others...
in real life:
Your users (or the users of your client) want to use a non-microsoft email client like webmail, but MUST be able to receive attachments packed in a winmail.dat file by badly configured Outlook, THEN you need tnef2mime. It converts transparently the proprietary winmail.dat AKA TNEF to a standard mime attachment. And, as a benefit it also converts uuencoded attachments, produced by old style unix mail clients.