There are a number of reasons that a literal Millennium must occur.
To reward the people of God: There are scores of promises scattered throughout the Bible, both in the Old Testament and in the New Testament, guaranteeing God’s people that they will receive rewards for faithful service (Matthew 16:27; Matthew 25:34; Revelation 22:12). And part of our heavenly reward will be to reign and rule with Christ upon this earth during the Millennium. Each of us will have opportunities to serve the Lord based upon our faithfulness in serving Him right now.
To respond to the prophets’ predictions: The prophets of the Old Testament predicted such a time (Psalm 72:11; Isaiah 9:7; Luke 1:32-33). And without the Millennium, none of those prophecies could be fulfilled.
To receive an answer to the disciples’ prayer (Matthew 6:8-13): One day, when Jesus returns, His kingdom will come and His will shall be done on this earth.
To reemphasize man’s depravity and the necessity of Christ’s death: During the Millennium, those faithful servants who survive the Tribulation will bear children, in whom the sin nature will reside, because the fallen human nature of man will not be eliminated until eternity begins at the end of the millennial kingdom. At the end of the thousand years, Satan will be released, and he will stir up a final rebellion against God just as he did in the Garden of Eden (Revelation 20:1-3, 7-8). That’s right—even though Christ is ruling and reigning on the earth during the Millennium, some will yet be deceived. This demonstrates just how deeply man needs a Savior. Man can never achieve righteousness apart from God.