The Nephite records would not have included the teachings of the prophet Malachi. The prophecies of Malachi were given approximately 430 B.C., almost two centuries after Lehi left Jerusalem.
These prophecies were considered so significant that the Father commanded that they be included in the Nephite scriptures. Inclusion of these words in the Book of Mormon serve as an additional witness of the truthfulness of the words of the prophets-“proving to the world that the holy scriptures are true, and that God does inspire men and call them to his holy work” (D&C 20:11).
Just as the Savior, during his ministry among the Nephites, quoted from Isaiah and Malachi, so did Moroni quote from them as he counseled Joseph Smith concerning the work of the Restoration (see Joseph Smith History 1:36-40).
This demonstrates the significance of these prophecies about the work of the last days in preparation for the Savior’s second coming. (For a more extensive discussion of Malachi’s teachings in the Book of Mormon the reader should refer to Kent P. Jackson, “Teaching from the Words of the Prophets,” in Studies in Scripture 8: 196-207.)