Laws are made by the legislative branch (Congress) but it is the Judicial Branch (the courts) who interpret the law. The courts work under the principle of "stare decisis" -- the legal principle by which judges are obliged to obey the precedents established by prior decisions. The words originate from the Latin phrase Stare decisis et non quieta movere, "Maintain what has been decided and do not alter that which has been established".
Lawyers need to know when a case has already been cited, and whether it is still "good law" -- case law that has not been overturned or superseded by the courts. In order to do this they "Shepardize" the case.