From verse 11 though the end of the chapter is Mormon’s conclusion to this event. Even though he was using Nephi’s record, Mormon could not help but make certain that the important lessons would be underscored. He begins his discussion of the moral of these events in verse 11. This is the reason that there is a repetition of the idea that the more righteous people were saved and their cities were not destroyed. That is the moral of the story.
It was also part of history, but as a historical event, it suggests that the more righteous people had remained in the more northern part of Nephite lands, and those lands were spared the destruction on both the south and the north. Geologist Dr. Jerry Gover examined the fault plates in the proposed region of Mesoamerica and suggests that the nature of the underlying geology explains both the destruction on the south, and the reason that Bountiful was not affected by that destruction, being on more a solid underlying geology.