Before Aaron and his sons could serve as Israel’s priests, they had to be washed and clothed.
Once the tabernacle and its furnishings were prepared, it was time for Israel’s Levitical priesthood to begin.
But before Aaron and his sons could serve as priests, they had to be properly clothed and cleansed:
1 Now the Lord spoke to Moses, saying, 2 “Take Aaron and his sons with him, and the garments, the anointing oil, a young bull as the sin offering, two rams, and a basket of unleavened bread; 3 and gather all the congregation together at the door of the tabernacle of testimony.”
4 So Moses did as the Lord commanded him, and the congregation was gathered together at the door of the tabernacle of testimony. 5 Then Moses said to the congregation, “This is the word the Lord commanded us to do.” 6 So Moses brought Aaron and his sons and washed them with water. 7 He put the tunic on him, girded him with the belt, clothed him with the robe, and put the ephod on him; and he girded him with the intricately woven band of the ephod, and with it tied the ephod on him. 8 Then he put the breastplate on him, and he put the Urim and the Thummim in the breastplate. 9 He also put the turban on his head; and on the turban, on its front, he put the golden plate, the holy crown, as the Lord commanded Moses.Leviticus 8:1-9 OSB
Then, the altar, utensils, and tabernacle had to be anointed for sanctification:
10 Moses then took the anointing oil, 11 and he sprinkled some of it on the altar seven times, anointed the altar, and sanctified it, and all its utensils, and the laver and its base, to sanctify them. He also anointed the tabernacle and everything in it, and sanctified it.Leviticus 8:10-11 OSB
Then, Aaron was anointed.
12 Moses then poured some of the anointing oil on Aaron’s head and anointed him, to sanctify him.Leviticus 8:12 OSB
Lastly, Aaron’s sons were dressed.
13 After this, Moses brought Aaron’s sons and put tunics on them, girded them with the belts, and put hats on them, as the Lord commanded Moses.Leviticus 8:13 OSB
These actions were necessary because Aaron and his sons were sinners. Sinners cannot simply march into the presence of GOD on their own. GOD provided these mechanisms by which sinful men could be considered worthy to perform priestly duties on behalf of the nation.