Rabbi Moshe Chaim Luzzato (Ramchal) From Derech Hashem.
The significance of Yom Kippur is that God set aside one day for Israel, when their repentance is readily accepted and their aveiros (sins) can easily be erased.
Kapora actually erases aveiros, which is way beyond waht Selicha (forgiveness) and Mechila (pardon) accomplish. Yom Kippur is the one day set aside for this Kapora and the Ramchal points out that they can easily be erased on this day.
This rectifies all the spiritual damage caused by these aveiros, and removes the darkness that strengthened itself as a result of them.
Every time we do an aveira (sin) much spiritual damage is caused and darkness (which includes the concealment of G-d) is strengthened. But the Kapora of Yom Kippur corrects the spiritual damage and removes the resulting darkness resulting from our sins.
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