Answer:
The Supreme Court's main purpose is to interpret the law and defend the Constitution. Often they must hear the cases of lower federal courts. They must check that laws follow the Constitution.
When the Supreme Court rules on a constitutional issue, that judgment is virtually final- its decisions can be altered only by the rarely used procedure of constitutional amendment or by a new ruling of the Court. However, when the Court interprets a statute, new legislative action can be taken.
Hope that helps. x
