This verse needs to be read in conjunction with the final clause of the preceding verse:"...and we know that the things which we write upon plates must remain." It is the contrast between the permanence of the plates and the impermanence of other media that is the point Jacob makes.
Anthropological: Most examples of writing in Mesoamerica come from a later period that Nephi and Jacob - indeed, from a later period that most of the Book of Mormon text. There are indications that writing was available from Olmec times (pre-Lehite, the time period for the Jaredites). On what might a Mesoamerican have written? We know of at least our types of media, stone, pottery, animal hides and bark paper (amatle). There are some writings on stone from early periods, and much more from later periods. While stone is the most permanent of the other options, we have broken stone texts and precious few from the earliest periods. Stone is also difficult to work on, and therefore the greater quantities should have been on the other more accessible, media. Each of them has their own preservation problems in Mesoamerica, and the dearth of early written materials - those for the time period prior to the rise of the Maya use of glyphic writing, are sadly scarce. That very scarcity underlines Jacob's contention that all other forms of writing would pass away, and only the difficult writing on the plates had the chance of permanence.
Scriptural: Jacob tells us something of the intended audience. Jacob tells us that they are intended for their children, and we understand that. He also says that they are for their "beloved brethren." Who? From Nephi's writings it is absolutely clear that Nephi supposed that his writings would go not only to his own lineage, but also to the descendants of Laman and Lemuel. Jacob is repeating that assertion. What is most interesting is the description of the Lamanites as "beloved brethren" by the same man who was, in the previous chapter, discussing their unrighteousness and filthiness.
We have in this declaration Jacob's personal opinion. In the previous chapter he was using common prejudices to highlight his point. Here we have his real feelings for them. Jacob could not help but retain some care and concern for them. Jacob more than most who would now be alive in land of Nephi would feel a very real kinship with the Lamanites, and remember a time when they were together as a family. It is those feelings he expresses here.