It looks like a lot of unnecessary finery: chestpieces, ephods, robes, patterned tunics, turbans, sashes, and jewellery. It was to be made with fine linen cloth, gold thread, and blue, purple, and scarlet thread, and also previous stones and gold. Why so much effort for desert nomads?
Firstly, it was all plunder from the Egyptians. The Israelites had done nothing to earn the materials they donated for this cause, so having richly dressed priests wasn’t really costly.
Secondly, everything had a purpose.
For example, Aaron, as high priest, was to have an onyx stone on each shoulder:
Exodus 28:9-12 Take two onyx stones, and engrave on them the names of the tribes of Israel. Six names will be on each stone, arranged in the order of the births of the original sons of Israel. … Fasten the two stones on the shoulder-pieces of the ephod as a reminder that Aaron represents the people of Israel. Aaron will carry these names on his shoulders as a constant reminder whenever he goes before the Lord.
The chestpiece was to be worn every time Aaron wanted a decision from God. It had 12 stones one it, each with the name of one of the tribes of Israel engraved on it:
Exodus 28:29 In this way, Aaron will carry the names of the tribes of Israel on the sacred chestpiece over his heart when he goes into the Holy Place. This will be a continual reminder that he represents the people when he comes before the Lord.
The finery was a reminder to Aaron of the responsibility he carried every time he met with God. It was also a symbol of the same to the Israelites.