Yom Kippur is a time when we stand before Hashem like malachim. This is in contrast to Shavuos, when we celebrate receiving the Torah specifically because we are humans, not malachim. When Moshe went up to receive the Torah, the malachim gave him gifts, the most precious of which was the ketores, the secret of stopping the malach hamaves (angel of death). The ketores is central to the avodah on Yom Kippur. Adam's sin brought death into the world through four senses, but the sense of smell was not involved. The sense of smell connects us to a reality that we cannot touch, something higher and finer, helping us to connect to the spiritual and our relationship with Hashem.