Deuteronomy 18:7

then he shall minister in the name of the Lord, his God, as all his brethren the Levites do, which stand there before the Lord. It seems that only a part of the Levites was actively engaged in the ministry of the Sanctuary, probably because their ministry was regulated by courses. The men here spoken of were not on duty, not under obligation to be present at the Sanctuary, but were simply zealous to be active.