In the beginning was the word, and the word was with god, and the word was god.  It is this word, sometimes referred to as logos, sometimes as Jesus, that has existed eternally and did become incarnate for the love and mercy within himself to be applied to us when we deserved it the very least.  
He (Jesus, the word) took on corruptible human flesh and indwelled it himself, though he is incorruptible. He laid down his riches, his kingdom, his throne, and for mankind became a slave. He willingly and knowingly allowed himself to be offered up as a tribute to defang and defame death. He did so in one of the most imaginably torturous ways to have occurred in human history. He laid himself down to be lifted up. He fought not the fate of crucifixion, for redeeming the fate of man was his mission.
Why would the ultimate goal of the pre-existing God of the universe be to redeem a creation that had defied him? Why, when betrayed, was the ultimately powerful God compelled to give up his ultimate powers to recapture our affections and our fates from what our defiance necessitates?
There are natures of God, which we, as humans, do not fully understand. We understand God to be just. We understand him to be merciful. We understand him to love. Then, we look upon the world and we see death. We see corruption. We see the suffering of innocents at the hands of the wicked. We see terrible natural disasters destroy entire nations.  There appears here to be a confliction of God and his nature(s) with the reality of the state of the world.  
According to Athanasius, these conflicts result from the fall of man. The moment man gave up his purity to corruption, by choice; the entirety of creation began to follow him into evil, into non-being.  Also according to Athanasius, these conflicts are necessary for God to remain consistent unto himself.
God found himself that day of our betrayal, with a conundrum. He was just, and he had allowed our freedom out of love, so that we may, by choice, truly love him, as opposed to , by lack of choice, robotically obey him. We abused that choice, and though he loved us dearly, we struck a bargain with death. It was a debt that would destroy us. It was God’s love that would not allow him to see us destroyed without intervention.
God had spoken and he, being truth, could not reverse his words. To do so would have been to falsify his entire nature. God also loved, and he could not allow the object of his desire to be so ruined as to have never existed. He could not allow mankind to die and remain true to his own being and nature.
And so, God, in the only way possible, paid our debt. He destroyed death by himself coming as the incorruptible to dwell within corruptible flesh. Upon the death of the body he indwelled he, being incorruptible within the body, could not pass away. And so, after three days, with death’s sting in hand, he rose. He was the only one able to become a thorn in death’s side. Thusly he demonstrated his power over death and the payment of the debt of mankind in an acceptable sacrifice.
It was for mercy, it was for justice, it was for love, and it was for grace, that he became incarnate. It was from before time, from the beginning of creation, from the birth of man at his hands, from the moment his breath filled Adam’s lungs, it was from then, that it began. In creation, it is the incarnation and the resurrection that so clearly paint a portrait of God himself, and just how he loves man.  But the incarnation is an image of the character of God, acted out towards his creation. Just as God’s creation is a portrait of himself in some way, so is his rescuing and redeeming it.
He loved us so well he would bleed. He loved us so well he would die. And still, though men ignore, or rail against him, he would love them. He does not turn away. He does not turn his back. If centuries of men turned vile could not repulse him from his love, how much less could another day? He is everlasting, unchanging in his love.
There is a thread of scarlet, weaved from the very moment we fell, up until the day we shall be redeemed in the greatest and last resurrection. This thread from death, to life, through love, is Jesus. It is the word incarnate. His incarnation is a stamp of lipstick that seals a love letter to humanity.
So why it is that he came? He came to live, to be tortured and to die, yes. But what is most important is this: He came for us. And why then did it have to be that he came in such a way? We are men, and so like us, a man was required to pay man’s debt. We owed a debt to death, and so, like we had to, he also had to die.  
So what did his coming accomplish? His coming accomplished restoration of us to a place of life. In a sense, the restoration of the image of god within us to its full manifestation has been replaced within the proper space within us: Though this manifestation will not be finally consummated until our glorification.  It accomplished all it intended, and it intended our full resurrection, in all senses of the word.  We are resurrected unto life, unto intimacy with god, unto hope for a future, unto the loss for words at his love for us. We are resurrected unto eternal paradise with the God-man who loves us most.
This is the reason for the incarnation. We need be not silent about it.