Nicely put by Judge Posner:
The supreme court is a political court. The discretion that the justices exercise can fairly be described as legislative in character, but the conditions under which this "legislature" operates are different from those of Congress. Lacking electoral legitimacy, yet wielding Zeus's thunderbolt in the form of the power to invalidate actions of the other branches of government as unconstitutional, the justices, to be effective, have to accept certain limitations on their legislative discretion. They are confined, in Holmes's words, from molar to molecular motions. And even at the molecular level the justices have to be able to offer reasoned justifications for departing from their previous decisions, and to accord a decent respect to public opinion, and to allow room for social experimentation, and to formulate doctrines that will provide guidance to lower courts, and to comply with the expectations of the legal profession concerning the judicial craft. They have to be seen to be doing law rather than doing politics.