Exodus 28:43
And they shall be upon Aaron, and upon his sons, when they come in unto the tabernacle of the congregation, or when they come near unto the altar to minister in the holy place; that they bear not iniquity, and die: it shall be a statute for ever unto him and his seed after him.
God distinctly specified the exact style of garments Aaron and his sons must wear when entering the tabernacle of the congregation or coming near the altar to minister in the holy place, requiring dress in accordance with their sacred work. The prescription was part of His directions that everything connected with approaching Him be orderly, clean, and free from impurity. Exact directions were given for the garments worn by those ministering in the tabernacle, making prescribed clothing part of their duties when they entered or approached to minister.
Joined to washing hands and feet at the laver placed between the altar and the congregation in sight of the people, it taught that every particle of dust must be put away before coming into the presence of God. The reason is that God is so high and holy that failure to comply with these conditions would cause the priests to bear iniquity and die. Their care in appearance and cleanliness honored God and helped the people recognize His holiness and the sacredness of His service, while carelessness could weaken the people's reverence.
Aaron's robes were symbolic, and the care given to them shows that God has a preference concerning the dress of those who serve Him, whose clothing was to represent Him. Likewise, Christ's followers today should dress as His representatives.