Exodus 28:2 "And you shall make holy clothes for Aaron your brother, for honor and for beauty"

An employee's uniform is designed to suit his place of work.

The guards at Buckingham Palace wear an outfit much different from a guard at city hall.

We are commanded to make the priests' clothes beautiful and dignified, reflecting the glory of the Beit HaMikdash in which they work. The priest must wear these clothes while performing the service.