These blockers prevent persons from visiting (deliberately or otherwise) Web sites containing particular content, such as violence, hatred, casino gaming, and Barry Manilow. (Kidding there. I think.)
The main purpose of these products is to protect children from accidentally or deliberately viewing content intended only for adults. They also prevent you from accidentally bumping into unpleasant sites: Try misspelling a Web site’s name and see where it takes you. On second thought, don’t try it.