Or HaChaim on Leviticus 8:2/אור החיים על ויקרא ח׳:ב׳

1 קח את אהרון, "take Aaron, etc." This paragraph was revealed to Moses before the Tabernacle was erected, and the reason it was not recorded up until now is to tell us that all the laws pertaining to the various sacrifices were revealed before the details of the consecration rites were revealed. Aaron and his sons had to become priests in order to perform any of the other services in the Tabernacle; this did not happen until Moses had taught him all the details about the various sacrifices and he had undertaken to perform every rite in accordance with what the Torah demanded. Only after he undertook to do this did G'd tell Moses to anoint him and his sons as priests. If this had not been the case, some Israelite might have decided to offer a sacrifice to G'd on the altar in front of the Tabernacle and he would not have known how to proceed.