When Benjamin says: “were it not for these things,” he means the plates of brass. They were acquired by miracle and represent both the connection to the ancient house of Israel and also to their authority to teach Jehovah’s commandments. Without the scriptures, the Nephites would not have been able to use the unchanging text to clarify and interpret the law according to their circumstances. They would have had to adopt more and more of the customs of the surrounding lands, something that had already happened to the people of Zarahemla before the Nephites merged with them.
Benjamin speaks first of the plates of brass, but then added the plates of Nephi. For Benjamin, this was the large plates. Even though he had the small plates by this time, the record of the kings and, therefore, the one most on the mind of kings, was the large plates of Nephi, not the small plates. There is little evidence that the small plates were read by subsequent record keepers, and less evidence that they quoted from them. We remember that Mormon was surprised to find them. He did not appear to know that they existed.
The admonition to study the scriptures diligently and to keep the commandments will become very relevant when we read Benjamin’s discourse. Mormon adds these instructions to Benjamin’s sons, precisely because they fit into the context of the reason for the public discourse.