The GNU Free Documentation License - like the GPL - provides a way for authors to contribute their works to all humanity. Unlike works released into the public domain, GFDL and GPL protected works do not become proprietary when a derived work is created from them: the derived work is also protected by the same license. So an author never experiences the pain of seeing an improvement on his work taken out of his grasp. The derived work is as much his as it is anyone else's.
The GFDL was an afterthought to the Free Software Foundation's GNU General Public License and was originally created solely with software documentation (such as on-line help manuals) in mind. Its biggest single user has been the mammoth and still burgeoning Wikipedia project.