All of the books of the Prophets and the entire body of the Sacred Writings are to be annulled in the Messianic Age - with the exception of the Book of Ester. It, like the Five Books of Moses and the laws of the Oral Tradition, will never be annulled.
Torah calls for preparation. The Patriarchs preceded the giving of the Torah, and, as the foundation of the Jewish people, fulfilled the entire Torah even before it was given. This measure was taken as a kind of preparation for receiving the Torah.