As much as we all love this story, but that's the story of what, not the story of why. It doesn't tell us the purpose of Christmas. Everything has a purpose, right? Why is the purpose of Christmas?
The harsh reality is that the baby was born to die. Instead of trying to explain, I'll let the Bible speak for itself. The following passage so beautifully summarizes the Christmas story.
"Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. Wherefore God also hath highly exalted him, and given him a name which is above every name: That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father." Philippians 2:6-11
There was a redemption because there was a resurrection.
There was a resurrection because there was a death.
There was death because there was a life.
There was a life because there was a birth.