The Lord instructed Nephi to make two sets of plates. Nephi said, “Wherefore I did make plates of ore,” that is, the first, or large, plates of Nephi. The first set recorded Lehi’s ministry, his genealogy, the family’s travels, and other historical events, especially political and military history. The second set recorded Nephi’s ministry, prophecies, and other matters of a more sacred nature. He continued, “And I knew not at the time when I made them [the large plates] that I should be commanded of the Lord to make these plates [the small plates].” He then certified, “I do not write anything upon [the small] plates save it be that I think it be sacred.” As noted already in the commentary at 1 Nephi 9, the prophet didn’t know all of the Lord’s wise reasons for making two different records, but he was willing to be exactly obedient.
Note the wisdom in engraving on metal plates so the record would endure far longer than ancient papyrus and even parchment (animal skin) or our modern paper and plastic materials on which we engrave or “burn” records.