Answer:
They protect justices from political pressure.
Explanation:
The justices' main role is to evaluate laws and decide on cases between two or more parties according to the law. They hold hearings, discuss the cases and come to a conclusion. And in order to impartially carry this task, they are given lifetime appointments so that they do not feel pressured to gain people's vote when running for re-election and the pressure to determine cases based on what people would like. Instead, they are expected to decide cases objectively, according to the law.