Read Hebrews 2:10-18.
“Therefore, since the children share in flesh and blood, He Himself likewise also partook of the same, that through death He might render powerless him who had the power of death, that is, the devil, and might free those who through fear of death were subject to slavery all their lives. Therefore, He had to be made like His brethren in all things, so that He might become a merciful and faithful high priest in things pertaining to God, to make propitiation (atonement) for the sins of the people.” (2:14-15, 17)
Every year, once a year, the high priest of Israel would stand before the people and slaughter a bull to offer its blood before the Lord as a sin offering for the people. The high priest would offer blood for two reasons: first, to atone for his own personal sins and, second, to atone for the past sins of the nation. The Day of Atonement, Yom Kippur, was the day in which the fate of the nation was placed in the hands of one man — a mediator — as he went before God.
Jesus Christ had to take on flesh and blood so that He could be a mediator between men and God the Father, the Righteous Judge of all. The people He was to represent on the cross needed to be people He fully related to in order for Him to be a substitute for them.
The logical (if it can be called logical) thought process went like this: Man was sinful so blood had to be shed to atone. No mortal man could be sinless and, thus, be a capable substitute. Only God was holy. So God had to shed His own blood. To do this, He had to take on flesh and blood just like a man. He had to be made like us in every way. So He decided to take on flesh in the same way normal men did — from conception in the womb. In that way, he fully experienced the depth of humanity (and also the death of humanity).
Even more, the author of Hebrews tells us in chapter 2, Jesus chose to be like us in our day-to-day experiences so that He could relate to us in our day-to-day experiences. He was tempted in every way yet was without sin. He overcame so that we could overcome through following His example.
Jesus Christ is not only our Lord in His eternal deity but He is also our brother in His humanity. He considers us brothers, too, as crazy as that sounds. The only way He can do this is to be like us in every human way, and the incarnation accomplished that in a mysterious, grand way.