God is eternal and he is spirit. He has no need to reproduce as mankind must in order to continue the existence of our race. He is not Zeus, of the Greek pantheon, producing offspring here and there. God is eternal and he is spirit. No aspect of him needed to be created.
John tells us that the Word became flesh. This aspect of God referred to as the Word, was poured into human flesh in order that he might come into this world as all human flesh must, by being born of woman. In doing so, God fulfilled his promise to Eve, that her offspring would one day crush the serpent which had deceived her.
We, as human beings, will probably never fully understand how God can be simultaneously and cohesively a Father, a Son (Word), and a Spirit. We will never truly understand how the Word of God was able to take on flesh in the first place; how an infinite God became finite flesh for 34 years of human history while he was yet still our Father in heaven.
While this concept is interesting (and mind blowing) to contemplate, in truth it is not an important question. The vital question is: Why did he feel it necessary to do so?
First, it is because he is a just God. The world stands guilty before him of crimes against his being. We stand guilty of rebelling against the God who was before all things and created all things. In the time of judgment, he must pronounce us guilty and mete out our just punishment.
Yet, he is also a loving God. He is a gracious God, inclined to extend his mercy to sinners. He is slow to be angry with us and reluctant to mete out his just judgment. His desire is to offer his forgiveness and give us a second chance. In order to do that, he had to wipe the slate of the sins written out against us clean. He needed those sins to be paid for in advance of our judgment day so that on that great day we might righteously go free.
For this cause, he needed the ultimate sin offering to be sacrificed. There must be a day of vengeance, yet he has also determined a year of redemption.
“For the day of vengeance is in mine heart, and the year of my redeemed is come. I looked, and there was none to help; and I wondered that there was none to uphold: therefore mine own arm brought salvation unto me; and my fury, it upheld me.” (Isa. 63:4-5 KJV)
He looked, and there was none to save us. So he sent himself.