Rashi on Exodus
וכפר אהרן AND AARON SHALL MAKE EXPIATION — This refers to the placing of the blood upon the horns of the altar.
Rashbam on Exodus
אחת בשנה, on the Day of Atonement. This was confirmed in Leviticus 16,18 ויצא אל המזבח אשר לפני ה', “he shall go out to the altar that is before Hashem, etc.” On all the other days of the year the golden altar was used only for incense burning.
Siftei Chakhamim
For the uncleanness of the mikdosh and its holy things. I.e., the bull and goat of Yom Kippur atone for the following: 1. The kohein knows he is unclean and accidentally enters the mikdosh, thinking it is a regular building. 2. The kohein knows he is unclean and accidentally eats korbonos, thinking it is regular meat. 3. The kohein knows it is the mikdosh but is unaware of his uncleanness. 4. The kohein knows it is korbonos but is unaware of his uncleanness.