These two verses are not only their own chapter in our current edition of the Book of Mormon, but they were also their own chapter in the original 1830 edition. They serve as the conclusion to the book of 3 Nephi, and more importantly, to the highlight of Mormon’s text: the appearance and teaching of the Savior in Bountiful after his resurrection.
Mormon was a disciple, which our modern terminology would call an apostle. At the end of this book he adds an apostolic conclusion. He speaks to those who will read what he is writing, and tells them that they are to “hear the words of Jesus Christ, the Son of the living God.” Even though the words that Jesus commanded were those that Mormon was to write, Mormon made sure to declare that they came through the authority of the Christ.
Mormon knew that the Book of Mormon would come first to the gentiles, and so he addresses them here (he will address the Lamanites at the end of his eponymous book). The Gentiles are called to repentance, and to ”be numbered with [the] people who are of the house of Israel.”