Leviticus 16:6

And Aaron shall offer his bullock of the sin-offering, which is for himself, and make an atonement for himself and for his house, the term here apparently including not only the immediate family of the high priest, but the whole order of priests. Since all the priests were representatives and mediators of the people in their dealings with the Lord, it was necessary that their own sin, first of all, be covered and expiated in the sight of God.