The first reference we have to the purpose of the small plates comes in 1 Nephi 9:5: “Wherefore, the Lord hath commanded me to make these plates for a wise purpose in him, which purpose I know not.” Nephi says nothing more about the purposes here, but in 1 Nephi 19:3, he says, “the things which were written should be kept for the instruction of my people, who should possess the land, and also for other wise purposes, which purposes are known unto the Lord.” While Nephi is unsure of all of God’s purposes, he at least knows that the small plates will be used to instruct current and future Nephties.
Jacob doesn’t say much about the purposes, nor does Enos. By Jarom’s day, the purposes were understood this way: “Now behold, I, Jarom, write a few words according to the commandment of my father, Enos, that our genealogy may be kept … and … these things are written for the intent of the benefit of our brethren the Lamanites.” Note that we’ve switched from having the Nephites as the intended audience to the Lamanites. Also, genealogy has become important here, whereas Nephi only copied his genealogy onto the large plates. And Jacob, Enos, and Jarom don’t seem to have any sense of God using these plates for unknown, wise purposes.
By the time Mormon discovers these plates, he is well on his way to finishing his abridgment of the large plates. As recorded in the Words of Mormon, Mormon feels to include the small plates in their entirety alongside his abridgment. He says, “And I do this for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me…And my prayer to God is concerning my brethren, that they may once again come to the knowledge of God, yea, the redemption of Christ; that they may once again be a delightsome people.” Mormon admits that he does not know God’s purpose, but then quickly adds a prayer concerning his brethren — as if to say “I don’t why God is doing this, but I sure hope it’s to bring my brethren back.”