"And also that we may preserve unto them the words which have been spoken by the mouth of all the holy prophets, which have been delivered unto them by the Spirit and power of God, since the world began, even down unto this present time."
Not only did the plates contain the geneology of Lehi they also held the prophecies of the prophets from the beginning of time to the time of Lehi and Nephi. The desire of Lehi and Nephi was to preserve the words of the prophets as well as understand and share them.
As stated before we know that the words of the prophets was uncorrupted because of God's foresight in obtaining the plates. The words from the very beginning of the earth's time would be saved and used to help teach the children of this nation, helping them to understand God's ways better and to be available when God deemed the time right for the last days, which we now live in.
book metal have genealogy same have prophesy from prophet past. start time upto now. Lehi, Nephi want plate why? save word understand share same can.
Same say before word from Prophet change not, why? God see future can get book metal need. Word save from start and use how? Teach children country here, help understand God His way better can. Can have time right God finish decide, When? Now time us live. Finish.