An amicus curiae or friend of the court brief, is used by interest groups because: D) it allows the interest group to support one side of a case without taking legal action. An amicus curiae is someone who is not party to a case but that is there to provide information, expertise or insight usually in the form of a brief. In the case of an interest group an amicus curiae is a good way for an interest group to provide additional arguments on a case.