When the testimonies from different peoples flow together, it will demonstrate that Jehovah is constant in His dealings with His covenant people. The phrase that He is “the same yesterday, today, and forever,” comes from Hebrews 13:8, and appears eight times in the Book of Mormon, from Nephi to Moroni.
Because Jehovah is the Lord of the whole earth, and has covenant people scattered over all that earth, including the islands of the sea, He gives his word to His children, and they write. Therefore, it should not be assumed that the Bible would be His only communication to His children.