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The Mussaf portion of any prayers; Yom Tov, Rosh Chodesh, Shabbos represent the essence, the main point of that day.
On Yom Kippur, the essence of the service, the ikar is the avodah of the Kohen Godol. So much so is this the case that the Mussaf service centers around the order of the Kohen Godol’s service in the Kodosh Kodoshim (the Holy of Holies in the Beit HaMikdash). The service includes all of the preparations which the Kohen Godol makes prior to the service, the clothing he must wear at each step of the avodah, the number of times that the Kohen must bathe himself prior to each change of clothing and before each step of his service, the drawing of lots determining which goat is for Hashem and which for The Mountain of Azazel (the goat designated by lot to bear the burden of death to rectify B’nai Yisrael’s sins) and more. And by virtue of our being Hashem’s “most favored nation”, The Kohen Godol is to the Jews a paradigm of Hashem’s blueprint of what the Jews are to represent to the other nations of Mankind.
