The registration, or census, that Augustus decreed was for purposes of taxation. Luke mentions this census because he wants to explain why Joseph and Mary had to travel from Nazareth to Bethlehem while Mary was in the final months of her pregnancy. There is also some kind of irony in this story. Augustus, ruler of a great empire, is a tool in the hand of God the Almighty. His decree is the cause of the eternal ruler of this world being born in a small town, lying just south of Jerusalem.
2 This was the first registration when Quirinius was governor of Syria.