Why did priests need to wear garments?


These are the garments they are to make: a breastpiece, an ephod, a robe, a woven tunic, a turban and a sash. They are to make these sacred garments for your brother Aaron and his sons, so they may serve me as priests.  Exodus 28:4
            The garments visually set the priests apart from the others.  It was their uniform that gave them authority over the work they were doing.
            It also needed to be to breathe (linen) since they did hard work and to be able to be washed since it got messy (blood). 
            For the priests, it helped them to know that they were on-the-job, so they slipped into the role of priest every time they put the uniform on.