Wearing Weekday Clothing to Remember to Ask for Gashmius

Rav Bunim of Peshischa zy”a explains that the Kohen Gadol is on an extremely lofty spiritual level when he enters the Kodesh Hakedoshim on Yom Kippur and, therefore, is apt to forget to daven for the nation’s physical and material needs. Therefore, he is commanded to wear weekday clothing to remind him to pray for these things.