Why would China need North Korea to test its nuclear weapons? They have a long history of nuclear tests and are a member of the club so to speak. They never did

Why would China need North Korea to test its nuclear weapons? They have a long history of nuclear tests and are a member of the club so to speak. They never did the large tests that the Soviet Union or the US did, but I don't see why they would. No one needs a 1+ megaton warhead if you have an accurate delivery system.