Words of Mormon 1:7

"And I do this for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me. And now, I do not know all things; but the Lord knoweth all things which are to come; wherefore, he worketh in me to do according to his will."

Mormon had the Spirit with him. Because he had this companionship he knew to include extra plates in the records he had. This was essential to replace the 116 pages that were lost by Martin Harris. The Lord knew that he needed to preserve these extra plates, and he told Mormon this through the Holy Ghost. That Spirit still whispers to us and helps us fulfill God's will.