Scriptural: The words of the prophets can keep present the dealings of God with man, and bring them to our ready remembrance. Having such records allows us to better follow the will of God.
Anthropological: Benjamin provides some important information here that should be analyzed. He first indicates that the ability to read the records of the prophets has allowed them to continue to believe, in contrast to "our brethren, the Lamanites." In this particular case, however generalized the term Lamanite has become, it is beneficial to Benjamin's argument to make sure that he references the original Lamanites. He is thus creating a very wide gulf between current Nephite and current Lamanite with respect to religion, and he credits the difference directly to the Nephite retention of the sacred records. At the time of the historical separation of Nephite and Lamanite, the records would have been the brass plates, and whatever perishable record Lehi had made (both records appear to have gone with Nephi).
The Lamanites would be deprived of the brass plates, which deprived them of much of what we consider the Old Testament. They would have missed Nephi's writings entirely simply because Nephi does not begin them until after the split has occurred. Benjamin takes for granted the loss of true religion to the Lamanites, and places that blame on their inability to read the accounts (not because they could not read in this case, but because they do not have the text to read from).
As a second problem, Benjamin notes that not only have they lost their religion, but that they: "even do not believe them when they are taught them, because of the traditions of their fathers, which are not correct."
Benjamin's argument to his sons emphasizes the necessity of reading the plates (learning the language of the Egyptians!) so that they do not become like the Lamanites and lose their religion. What is missing in this discourse is the reason that the reading of the plates should be so essential. To understand what is behind this assertion, we need to make certain things clear.
First, the Lamanites would not have originally been illiterate, and they simply did not have the brass plates which went with Nephi. Secondly, the original Lamanites could have maintained their religion through oral means. The oral mode of institutional memory is quite capable of remembering tradition, particularly a tradition such as a religion that dictates action as well as belief. Indeed, the Lamanites do have an active oral tradition, but it is specifically this tradition of their fathers that prevents them from adopting the Nephite ways, even when those ways are taught to them.
Societies all change over time, and the literate ones simply have the record of those changes. Some variation in the traditions between Nephite and Lamanite might be attributable to written versus oral recollections of the Old Word religion, but that is in and of itself an insufficient answer. Had the Lamanites cared to remember, it would have been possible to remember much more than it appears that they did. Our knowledge of the operation of oral cultures suggests that it was cultural change more than a lack of a particular written record that caused the divergence between the two. The best explanation of the wide divergence was the greater degree of adoption of native culture by the Lamanites, an adoption so complete that the Nephites appropriated the name of the Lamanites as a generic label for all other peoples, for all of that "new" influence that was apparently to readily adopted by the Lamanites, the Zarahemlaites, and resisted with such effort by the Nephite prophets.