These two verses are essentially a repetition of the information from Ether 3:21. In that verse, this instruction is given by the Lord in the context of the meeting with the brother of Jared. Now we have a repetition of the information in a new context, which is Moroni’s description of the event rather than his repetition of the words of the Savior. Textually, this repetition comes because it is now Moroni discussing the information. These verses serve as the transition from what Moroni was taking from his sources and another of Moroni’s asides. In this case, the aside is triggered by the fact that it is Moroni who is the agent fulfilling the promise that the information should come forth after Christ had appeared in the flesh.