The incense burned on the golden altar was meant to be the most beautifully aromatic thing around. Why, then, was this foul-smelling ingredient, chelbona, included?
Sin leaves a "stain" on the spiritual aspect of the person. The highest levels of repentance may be so deep, that now a new level of goodness and holiness is reached, which could only be attained by repentance...
We might think it is sufficient if the Divine dwells only in the most spiritual part of our lives. This is why Moses was at first reluctant to accept the mirrors...