Halakhah for Yoma 170:12

It is from the roots of the commandment that it was from the kindnesses of God towards His creatures to fix one day in the year for the atonement of sins, with the repentance that they repent. As if the iniquities of the creatures would accumulate year by year, their measure would be full at the end of two or three years or more, and the world would be liable for destruction. And therefore He, blessed be He, saw in His wisdom - for the survival of the world - to fix one day a year for the atonement of sins for penitents. And from the beginning of the creation of the world (Bereshit Rabbah 2:3), He designated it and sanctified it for this. And since God, blessed be He, designated that day for atonement, the day was sanctified and received the power of merit from Him, may He be elevated, to the point that it aids atonement. And this is [the meaning] of their, may their memory be blessed, saying in many places (Yoma 85b), "Yom Kippur atones" - meaning to say that there is power in Yom Kippur, itself, to atone for light sins.