Nephi’s instructions to Jacob are quite explicit: There are two sets of plates, one primarily for the history of his people (but which clearly contained events of spiritual significance, since those plates were the basis for Mormon’s abridgment), and these smaller plates that have the more focused of recording the things of the Spirit: “Nevertheless, I have received a commandment of the Lord that I should make these plates, for the special purpose that there should be an account engraven of the ministry of my people” (1 Ne. 9:3).
Apparently, Jacob had not previously understood that the small plates had this purpose, presumably because Nephi had not seen fit to discuss his record-keeping function with his brother and because Jacob had not served as a record keeper until Nephi passed over the plates. Indeed, Jacob turned out to be a not particularly voluminous writer. Most of his text consisted of copying previously written records, rather than writing new text on the plates. This introduction is an exception.