Sometimes you want to send something without having your real name attached to it. For example, you are an official spokesperson for a company, and now you want to say something "off the record". Or maybe you want to discuss something that is going on in your company, without identifying yourself as the sender of the message. Perhaps you just don't want your boss to find out you are posting to alt.sex.bondage during working time.
Simply changing or clearing the "Real Name" field in your newsreader setup won't help you much in those cases. It's still trivial to trace a message back to its originating site, and of course the system administrator there can easily look up the sender's real name.
A better solution is to use an anonymous remailer. These systems either give you an anonymous address, to which other people can send you mail, which is then forwarded to your real address (this is sometimes referred to as a pseudonymous server), or they post or mail your message without any trace of the sender's name or address.