Why did the high priest have to undergo the entire ritual of cleansing?
The high priest had to undergo the entire ritual of cleansing because he was entering the very presence of God behind the veil on the Day of Atonement. Just as the Passover lamb had to be perfect and without blemish, so the priest had to be ceremonially cleansed so he would not die before the Shekinah glory.
Les explains that under the Old Covenant, the high priest could not enter the holy place behind the veil every day; he could enter only once a year on the Day of Atonement. The Shekinah glory dwelt above the mercy seat, so that place was the very presence of God. Aaron's sons had died when they offered before the Lord, so God warned Aaron not to come at all times lest he die. The priest's cleansing ritual therefore pictured the perfection required to approach God; just as the Passover lamb had to be without blemish, so the high priest had to go through the whole ritual of cleansing. This foreshadowed Christ, who fulfilled these pictures at the cross on our behalf.