Adam Clarke on Exodus 30:20

They shall wash with water, that they die not - This was certainly an emblematical washing; and as the hands and the feet are particularly mentioned, it must refer to the purity of their whole conduct. Their hands - all their works, their feet - all their goings, must be washed - must be holiness unto the Lord. And this washing must be repeated every time they entered into the tabernacle, or when they came near to the altar to minister. This washing was needful because the priests all ministered barefoot; but it was equally so because of the guilt they might have contracted, for the washing was emblematical of the putting away of sin, or what St. Paul calls the laver of regeneration and the renewing of the Holy Ghost, (Tit 3:5), as the influences of the Spirit must be repeated for the purification of the soul, as frequently as any moral defilement has been contracted.