Marbury vs Madison established the concept of judicial review.

“It is emphatically the province and duty of the Judicial Department to say what the law is…If two laws conflict with each other, the Courts must decide on the operation of each. So, if a law be in opposition to the Constitution… the Court must determine which of these conflicting rules governs the case. This is of the very essence of judicial duty.” Chief Justice Marshall, Marbury v. Madison, 1803

Their job is to rule on the case before them and to determine if it is, or is not Constitutional. By doing this they shape the law of the land. For years the SCOTUS held that segregation laws, providing that conditions were separate but equal, were not in violation of the Constitution. Years later they reversed their decision and held that separate but equal was not Constitutional, forcing states to change their laws. The regulation of marriage is not an enumerated power of the federal government, however in 1967 the Court ruled that laws forbidding interracial marriage were unconstitutional, and more recently they ruled that laws against same sex marriage were as well. As a result of their rulings in both examples, states had to change their laws to comply, thus making the Court the final authority as to the law. This is why I mentioned the political circus surrounding the confirmation process. Politicians want to ensure that people are selected who will most likely interpret vague or poorly written laws in their favor. An example of this would be those who want judges to use the militia clause of the 2nd Amendment to rule against private ownership of guns, or those who want judges to rule against abortion because the Constitutional rights of the child are being ignored.