God takes human birth again and again upon this earth, because there is a work to do and a road to be opened towards the future. Earth nature is full of ignorance and impurity. Every time the Divine takes a human form, it works to prepare the earth nature and carry it one more step towards the goal. Avatars destroy the old impurities and bring down the new truths.
Night is a shadow and not some original Truth. That is why it has not the ultimate absolute Power.
About Savitri | B1C2-10 Condition of the World (p.18)