The plates of Nephi were preserved so that the Nephites could know the mysteries of God. The mysteries were preserved for our day in the Book of Mormon. It is the truthfulness of the mysteries - "these things" - which we must ask God for confirmation through the Holy Ghost.
Seers are greater than prophets and know the mysteries of God. The mysteries on the 24 plates of Ether were included in the Book of Mormon so that we could know the mysteries therein.