Chapter 15 spoke of the destruction of Moab. When Isaiah was given this message, Moab was fairly small, but it was a productive nation. You may remember from our reading of the book of Ruth that this is where Naomi went when there was a famine in Canaan. And yet, Isaiah says that it shall be put to ruin.
The timing for the fulfillment of this prophecy is key. We learn in chapter 16 , verse 14 that it will take place within three years.
Isaiah was given this prophecy to give to his people in order to show them that there is One who is in control of the world and the kings in it, and that all who worship false gods are accountable to that One, the God of Abraham, Isaac and Jacob.
That the prophecy came true was a confirmation to all that Isaiah’s mission and his prophecies were true.