Making it so that Supreme Court justices have their job for life or until they decide to retire makes the Supreme Court apolitical body because the judges do not have to be voted on constantly by the citizens.
A Supreme Court justice who is constantly worrying about re-election campaigns may rule based on what he/she thinks will help them to get votes rather than worrying about accurately interpreting the meaning of the US Constitution. This life time appointment prevents political parties and influences from interfering with the judgement of these Supreme Court justices.
