In the first part of Nephi’s exhortation to his brothers, he focused on the need to fulfill Jehovah’s commandments. In this part, he emphasizes the reason for the commandment. Jehovah did not have them return for the plates of brass as a test of faithfulness. It was a mission with a critical objective, one that had to be accomplished.
The first argument is that the plates of brass would “preserve unto our children the language of our fathers.” We do not know if Nephi ever received a revelation that let him know of the fate of the people of Zarahemla, who had not preserved the language of the Jews. Assuming that Nephi had training in a scribal school, it would have been part of his training to see the records of the past as preserving both language and culture.
The second argument is that the plates of brass contain the teachings of the prophets. They contained the revelations of Jehovah’s relationship and covenants with his chosen people. Nephi certainly saw his New World people as inheritors of that covenant, and that had to have been present when he wrote of this story that had occurred decades before.
When we, as modern readers, approach the record that Nephi created, we must remember that he created it for a particular purpose and that it was created long after the events of 1 Nephi had ended. Thus, we may look to the world in which Nephi was writing to see the purpose in the selections he made, of which stories to tell, and how to tell them. In the case of dialogues, the lack of recording devices mean that all dialogue was recreated from memory. It is possible that when Nephi wrote of the need to preserve their language, he was living in the example of the reason why it was essential.