“Speak to Aaron, and to his sons, and to all the children of Israel, and say to them, This is the thing which Yahweh has commanded, saying,”
This brings out that, while Aaron and his sons were responsible for the priestly ministry, the details of the Law were still the province of Moses. It was he who had to convey to Aaron and his sons and the children of Israel the whole word of God. The priests' task would then be to apply that Law as it had been given to them by Moses.
In a sense this verse looks forward to the remainder of the book. It is ‘to Aaron, to his sons, to all the children of Israel' (only here, and in Leviticus 22:18 where it forms a transition from words to the priests to words to the people). This overall phrase covers both groups to whom Moses will speak in this last part of the book.