Literary: Isaiah restates the coming of the day of the Lord. This allows him to set up the next set of verses as poetic reinforcements of the first set that followed this phrase. The theme is the same, it is simply reinforced by the duplicated introduction.
It should be noted that verse 9 also specifically mentions the destruction of the "sinners" as opposed to just the Babylonians. Because Babylon can stand for the sinners of the world, this is not a surprising addition, but it does highlight the eschatological focus of this particular destruction. For whatever time or times there might have been some fulfillment in the past, this destruction is complete only in the second coming and final victory of the Messiah.