Why do the supreme court justices have a life term period?

Question

The justices of the Supreme Court of the United States is composed by chief justices which have a life-term period on that position after they are "elected".I have always asked to myself why some positions are by life-term, is there any reason like avoid corruption?

Answer 1

Article III, Section 1, of the U.S. Constitution provides:

“The judges, both of the supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.” 

The purpose of this clause is to assure the independence of the justices and all other federal judges. As Alexander Hamilton wrote in The Federalist No. 78, the assurance of judicial independence “is the best expedient which can be devised in any government to secure a steady, upright, and impartial administration of the laws.”