Reasons why ghostwriters work for others

You may find many articles, blogs, stories and books in the libraries both digital and physical, which are not completely written by the actual named author, but by the an anonymous person. This anonymous person is a ghostwriter and people hire his ghostwriting services to do their work or to complete their work due to any reason or maybe on purpose. In some cases, people hire these writers if they do not find themselves capable of creating perfect required content or books. Have you ever noticed any writer who remain anonymous and writes for another person?