There was more on them than there is in the Old Testament as we now have it. (1 Ne. 13:23.) The prophecies of Zenock, Neum, Zenos, Joseph the son of Jacob, and probably many other prophets were preserved by them, and many of these writings foretold matters pertaining to the Nephites. (1 Ne. 19:10, 21; 2 Ne. 4:2, 15; 3 Ne. 10:17.)
The value of the Brass Plates to the Nephites cannot be overestimated. By means of them they were able to preserve the language (1 Ne. 3:19), most of the civilization, and the religious knowledge of the people from whence they came. (1 Ne. 22:30.) By way of contrast, the Mulekites, who were led out of Jerusalem some 11 years after Lehi’s departure, and who had no record equivalent to the Brass Plates, soon dwindled in apostasy and unbelief and lost their language, civilization, and religion. (Omni 14–18.)
From prophet to prophet and generation to generation the Brass Plates were handed down and preserved by the Nephites. (Mosiah 1:16; 28:20; 3 Ne. 1:2.) At some future date the Lord has promised to bring them forth, undimmed by time and retaining their original brightness, and the scriptural accounts recorded on them are to ‘go forth unto every nation, kindred, tongue, and people.’ (Alma 37:3–5; 1 Ne. 5:18–19.)
(Bruce R. McConkie, Mormon Doctrine, p. 103)