A perfect sacrifice
The Godhead (God the Father and Jesus) possesses perfect, holy character that governs their existence. The moral laws revealed to mankind (e.g. the Ten Commandments and their expansion in the New Testament) are intrinsic to their eternal character and divine nature. They reflect their existence as the embodiment of perfect love. The principles they live by, when violated (sin) by others, brings separation from God and eternal death (Romans 3:23). As the source of these laws, only the Godhead can redeem anyone from the penalty of breaking them.
The Being who was born Jesus Christ was the Creator God of the Old Testament. He is the one who, through Moses, gave God's laws to ancient Israel. Since he was the originator of the Law for humanity, He also could take in His own Person the penalty resulting from that law, and stand in humanity’s place for it.
The Messiah had to be born of a woman and be like us in order to save us. He had to become human in order to die, and to give up His life temporarily, so that believers may live eternally. As the Creator of all things (John 1:1 - 3, Colossians 1:16), including man, His life is worth more than all the humans who have or will ever live. His sacrifice, therefore, completely pays for and covers all human sins either past, present or future (Hebrews 2:9, 10:10, 12, 14).
A merciful High Priest
14. Having therefore a great High Priest, Who has passed into the heavens, Jesus the Son of God . . . 15. For we do not have a high priest who cannot empathize with our weaknesses . . .
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 . . .
11. Now every high priest stands ministering day by day, offering the same sacrifices repeatedly, which are never able to remove sins; 12. But He (Jesus), after offering one sacrifice for sins forever, sat down at the right hand of God (Hebrews 4:14-15, 7:25-26, 10:11-12, HBFV throughout unless stated).
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)
1. Jesus spoke these words, and lifted up His eyes to heaven and said, "Father, the hour has come; glorify Your own Son, so that Your Son may also glorify You; . . . 4. I have glorified You on the earth" (John 17:1, 4)
Jesus, the God of the Old Testament, 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)
43. 'It is necessary for me to preach the gospel of the kingdom of God to the other cities; for this is the reason that I have been sent' (Luke 4:43)1. Then He (Jesus) called His twelve disciples together and gave them power and authority over all the demons, and to heal diseases; 2. And He sent them to proclaim the kingdom of God . . . 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 9:1-2, 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)
He has obtained a supremely more excellent ministry, as much greater as the superior covenant of which He is also Mediator, which was established upon superior promises (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.
34. Jesus spoke all these things to the multitudes in parables, and without a parable He did not speak to them; 35. So that it might be fulfilled which was spoken by the prophet, saying . . . (Matthew 13:34-35)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)
Not only did Christ 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!