The high priest was required to change into special garments of simple linen in order to enter the Holy of Holies.
Becoming a New Person
וּפָשַׁט אֶת בִּגְדֵי הַבָּד אֲשֶׁר לָבַשׁ בְּבֹאוֹ אֶל הַקֹּדֶשׁ וְהִנִּיחָם שָׁם: (ויקרא טז:כג)
[Aaron] must remove the linen garments that he wore when he entered the Holy [of Holies], and leave them [in the Tabernacle]. Leviticus 16:23

The linen garments worn by the high priest on Yom Kippur must never be worn again. The reason for this rule is that when a person repents sincerely, he or she is transformed into a new person. Inasmuch as the essence of Yom Kippur is repentance, the garments used by the high priest to perform the Yom Kippur rituals had to be new each year.1