This arrangement was made in obedience to a divine commandment, for a wise purpose, which, however, was hidden from Nephi. God does not always—perhaps seldom—explain the reason why he gives a commandment. A loving child does not ask for reasons, when God speaks.
When Mormon had finished his abridgment from the political history of Nephi, down to the reign of King Benjamin, he found, after research, the other plates of Nephi, “which contained this small account of the prophets, from Jacob down to the reign of King Benjamin, and also many of the words of Nephi.” Being much impressed with the contents of this record, he put the plates with his own work. “And I do this,” he says, “for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me.” (Words of Mormon, 3-7.)
This purpose did not become perfectly clear until during the summer of 1828. About that time Martin Harris had copied 116 pages of the translation, as slowly dictated by the Prophet Joseph, and, in his anxiety to convince his friends of the divinity of the work in which he and the prophet were engaged, he persuaded the latter to permit him to show them these pages. This part of the manuscript was lost. The loss would have been irreparable, but for the existence of the second set of plates of Nephi, which could be translated instead. (See 10:3-13)