Without the final two books of Paradise Lost, Milton would've failed in his original goal to justify the ways of God to men. He wouldn't have been in a position to explain God's reasoning and how it fits into his overall vision for humankind. Had the poem stopped before these two books, then we would've been left puzzled at the rationale behind God allowing sin and death into his creation. It would've made no sense at all, and many of his readers might well have believed that Milton's God had somehow set up Adam and Eve for a fall, and for no good reason.
But as Milton wants to give us a positive message, he sets out in book 11 to present a startling vision that fleshes out his notion of the Fortunate Fall. This is the idea that had humankind not fallen into sin, then there would've been no need for Jesus Christ, the only begotten Son of God, to save us.
From his vantage point on top of the highest hill in Paradise, Adam is able to see into the future, and what he sees is truly shocking. Before him, he beholds scenes from the Old Testament, in which successive generations of God's creatures indulge in war, theft, murder, and fornication.
Yet amidst all this brazen sensuousness there is still hope. And it is Noah who provides it. In the shape of this good and virtuous man, God preserves humankind while at the same time cleansing the world of sin with his great flood. As Adam watches the earth covered with a rainbow, he realizes that Noah is intended as an example of how one should behave, as a virtuous, obedient servant of God.
We as readers also realize, as Milton intended, that God's covenant with Noah was only possible because of the existence of sin and death in the world. In other words, in keeping with Milton's notion of the Fortunate Fall, humankind had to sin in order for good to prevail, whether in the shape of Noah, or later on, in Jesus Christ.