If a work is in the public domain, effectively no one can enforce any complaints against copyright infringement. Typically, this occurs because either the copyright has expired or because the creator of the work in question has specifically released it into the public domain.
I can only think of a few reasons a professional freelance writer might want to release material into the public domain (or use licensing like Copyleft or Creative Commons which effectively allows for uses of the work similar to public domain), such as to create interest in another product. For instance, a writer who has began to offer consulting services on a specific topic might release a white paper into the public domain in order to generate interest in the benefits of hiring her as a consultant.
Just because you cannot stop anyone from using your work if it is in the public domain does not mean that your work is wholly unprotected. For instance, if an academic researcher sites material you wrote, she is still obligated to cite your name correctly, and acknowledge that you were the original writer. Otherwise, it becomes an issue of plagiarism.