The brass plates play a vital role in the Book of Mormon. Nephi killed a man to obtain them, evidencing the importance of intellectual and spiritual literacy to the perpetuation of a civilization (see 1 Nephi 4:13, 18; Omni 1:17). They contained the five books of Moses; the prophecies of many of the Israelite prophets, down to and including Jeremiah; and a genealogy of Lehi and his family (1 Nephi 5).
In addition the brass plates were a more extensive scriptural record than the Bible as we know it (see 1 Nephi 13:23). We learn from Benjamin that at least some (or possibly all) of the brass plates were written in Egyptian (see Mosiah 1:4). They were the scriptures of the Nephites, and we suppose that multiple copies were made by the Nephites in order that all the people might have access to the written word of the Lord.