Nephi's instructions to Jacob are quite explicit about the nature of these plates. There are two sets of plates, one primarily for the history of his people (but which clearly contained things important to God. as those become the basis for Mormon's abridgement) and these smaller plates that have a more focused purpose. On these small plates Nephi emphasized the command he has received from the Lord to write on them particularly the things of the Spirit (1 Nephi 9:3).
It would appear that Nephi specifically gives Jacob this information. From this text it would appear that the purpose of the small plates is not something well known for Jacob. This is not particularly surprising, because Nephi has taken care of the records, and there is no indication of Jacob as a record keeper until he has these plates. Indeed, we will see that Jacob is not a particularly voluminous writer, with the majority of his text in this book coming from copied written records rather than text written directly to the plates (as this introduction appears to be).