The first goal sought by having Jesus born a human has a great deal to do with the uniqueness of his and God the Father's existence. They, as the only two Beings currently in the Godhead, have always, and will always, possesses perfect, holy character. The moral laws they revealed to mankind are intrinsic to their divine nature and are the embodiment of perfect love. Since it is they who maintain and enforce these rules throughout all things they created (whether physical or spiritual), nothing less than a member of the Godhead can redeem anyone from the penalty of breaking them.
The divine Being we refer to as Jesus, who was the God of the Old Testament, had to be born of a woman and be like us in order to save us. As the Person through which the Father created all things (John 1:1 - 3, Colossians 1:16), including man, His life is worth more than the lives of all humans. His sacrifice, therefore, completely pays for and covers all sins (Hebrews 2:9, 10:10, 12, 14). Since Jesus was the Giver of the Law to humanity, He also could take in His own Person the penalty resulting from that law, and stand in humanity’s place for it.
A merciful High Priest
14. Having therefore a great High Priest, Who has passed into the heavens, Jesus the Son of God . . .
25. Therefore, He (Jesus) has the power throughout all time to save those who come to God through Him because He is ever living to intercede for them. 26. For it is fitting that we should have such a High Priest . . . (Hebrews 4:14, 7:25-26, see also 10:11-12, HBFV throughout).
The High Priest in the Old Testament stood as a mediator between God and man. In addition to his regular duties at the temple, he had to enter the Holy of Holies once a year and sprinkle blood on the Ark's mercy seat in order to make an atonement for the people's sins. Jesus had to be born of a human in order to qualify as mankind's merciful High Priest in heaven. He had to experience the same temptations we do in order to be man's perfect intercessor with God the Father. On the Sunday after his resurrection (John 20:11-17) he ascended to heaven and offered his own blood as full payment for sin so that we can be forgiven.
Reveal and Glorify the Father
18. No one has seen God at any time; the only begotten Son, Who is in the bosom of the Father, He has declared Him (John 1:18)
27. All things were delivered to Me by My Father; and no one knows the Son except the Father; neither does anyone know the Father except the Son . . . (Matthew 11:27, see also John 17:1, 4).
Jesus was born in order to personally reveal the Father (Matthew 11:27). He was the Being who interacted with Adam and Eve, Noah, Abraham, King David, and others. It was therefore needful for him to reveal, personally, the other Being of the Godhead who was greater than he was (John 10:29, 14:28). By his miracles and perfect obedience, Christ drew man's attention to the Father, with whom he shared the same kind of character.
Preach the gospel and train others
14. Now after the imprisonment of John, Jesus came into Galilee, proclaiming the gospel of the kingdom of God (Mark 1:14)1. Now after these things, the Lord appointed seventy others and sent them two by two before His face, into every city and place where He Himself was about to come. (Luke 9:1, 10:1)
Christ needed to be born and conduct a ministry on earth in order to be the first one to herald the good news of God's Kingdom and to train others to do the same after his departure.
Destroy the works of the devil
8. For this purpose the Son of God appeared that He might destroy the works of the devil (1John 3:8)
14. Therefore, since the children are partakers of flesh and blood, in like manner He also took part in the same, in order that through death He might annul him who has the power of death - that is, the devil (Hebrews 2:14)
One of the "great" works of the devil was the deception of Adam and Eve. Their sin brought separation from God and death to the human race (Genesis 3:19, 1Corinthians 15:21-22). Jesus Christ needed to be born and be the perfect sacrifice for sin in order to make possible man's reconciliation with God. This reconciling with God, coupled with the forgiveness of sin upon repentance, nullifies Satan's efforts to thwart God's plan of salvation.
Establish the New Covenant
And the Lord, whom you seek, will suddenly come to His temple, even the MESSENGER OF THE COVENANT, in whom you delight (Malachi 3:1, see also Hebrews 8:6)
Jesus was born to both end the Old Covenant delineated in the Old Testament and establish a New Covenant with man made possible by his sacrifice. This new agreement with man promises, upon repentance and obedience, an eternal life full of love and happiness.
31. And after taking the twelve aside to Himself, He said to them, "Behold, we are going up to Jerusalem, and all things that have been written about the Son of man by the prophets shall be fulfilled" (Luke 18:31, see also Matthew 13:34 - 35)
Not only did Jesus fulfill roughly thirty prophecies on the day he was crucified, he also fulfilled many Old Testament prophecies regarding where and how the Savior would be born, the ministry of the Messiah and so on.
The Godhead's great plan for man is to ultimately create beings like themselves who have 100% free will and who choose to be righteous and obedient to their holy law 100% of the time. Jesus Christ was born more than 2,000 years ago in order to make this possible!