Mormon states that the prophets have testified. Now he gives the message to which they testified. Verse 15 returns to the prophets. Thus Mormon has also given us a bracketed text, with the prophets serving as the bookends, and the text itself describing what was prophesied.
The important information that Mormon sees is that the coming of the Messiah is associated with destruction. We know that these elements are associated with the Triumphant Messiah, and Mormon sees them as having fulfillment as well in the coming of the Atoning Messiah. The destruction of the wicked stands as a prophetic witness of the mission of the Atoning Messiah.