The Constitution of the United States promotes the value of independence for the Supreme Court by the practice of appointing Justices for life and not changing their salaries.
All the Justices are appointed by the President with the advice and consent of the Senate. They hold their office during Good Behavior which means during their lifetime. This way, they are not influenced by any political changes happening in the country and are not influenced by any political ideologies or leaders.
The salary of the Justices can not be decreased during their term either, which prevents them from looking for better paid positions and helps them to be free of financial pressure imposed by others in order to influence their decisions.