Mormon has made sure to enter the descriptions of the destructions because they were prophesied events, and Mormon wants to be sure that his readers understand that true prophecy is fulfilled. More important than the destructive events was the event that presaged them: the birth and death of Jesus in mortality. That was the message that the Nephite prophets had taught. Mormon makes certain that we understand that the prophecies are even older, tying them to the sacred records of the Old World through the prophets Zenos and Zenock, who he has previously cited in connection to these prophecies.
The destruction witnessed that all of the prophecies were true. However, one would remain, and that was that the Savior would come to this people.