In theory, the justices are protecting the intent of the Constitution.   There are nine appointees, steeped in education and experience designed to keep the constitution relevant to the modern world's challenges.   The SC was designed for exactly this purpose.   Lifetime terms mean the justice commitment is to the law, not the political interest of a single party.   If the constitution has to have a foundation, it cannot be subject to the whims of popularity.

The founders had no way of anticipating what we now refer to as global warming.    The modern supreme court will look at course cases for the last 200 years to look for precedence.   

At all times, the Supreme court is talking for a Constitution that can't speak for itself.   In theory.