"It is necessary for the Son of Man to suffer, and afterwards to enter in to his glory." Thus did Jesus state the necessity of his suffering. The question is, Why?

Jesus' death served as an exchange; a ransom. This purchased the race and gave Jesus sole ownership of it. The suffering part gave Jesus the transforming power that comes from Jesus' role as the great "sin offering".
In Hebrews 9, we read that the sacrifices of ancient Israel could not possibly make the priest who offered them feel like his conscience was cleansed. No matter how many cattle he slaughtered, he would always feel guilty about his own moral defects. But Jesus' sacrifice has transforming power. By accepting that Jesus was an innocent living being, voluntarily submitting to the fire of the altar, agreeing to spill his blood to be applied on behalf of both priests and the nation at large, Jesus fulfilled that law and established a process of conscience cleansing for all who participate. Now, anyone who is struggling with guilt can recognize that Christ satisfies God's justice, and we can boldly approach the throne of God for mercy. We do so, not in the strength of our own good deeds or penances, but in the strength of Jesus' one offering of himself for all time.
Jesus himself was "made perfect through sufferings". (Hebrews 2:10) The trial of his character, the sufferings He went through, added a depth of character and equipped him for the "high exaltation" (Philippians 2:9) that God intended for Him as a result of his ordeal. This was one of the joys that were set before Jesus, (Hebrews 12:2) which helped him endure the incredible pain of the cross.
This concept is expressed in different words in Hebrews 5:8, where it says that Jesus "learned obedience by the things that he suffered." The Bible thus teaches that the Son of God did not start out omniscient or all-knowing, but instead had to learn some things by experiences which the Father asked him to undergo. In Hebrews 5:9, we are told that by being "made perfect" through those sufferings, Jesus thus became equipped to become "the Author of eternal salvation unto all them that obey Him." That means anyone who obeys Jesus, follows in his footsteps of sacrifice and service, through faith in his blood, will join him in the great and glorious future of the Church of Christ -- a future that is truly glorious because it involves sharing that great joy with the rest of the world of mankind, who will be in need of lessons of obedience as they emerge from their graves. (John 5:28,29, compared with Isaiah 26:9 and Psalm 96. Judgment means correction and instruction in righteousness.)