TOPICS for CHAPTER 15:
There are songs in this judgment in the Revelation. This maintains the general Revelation pattern of songs before judgments. In this specific case, the songs of Moses and the Lamb prepare for the final series of judgments, the BOWLS.
Furthermore, from Revelation 8:13, these BOWLS constitute the third and most awful of the three WOEs that were announced to the inhabitants of the earth just before the fifth TRUMPET. When the last BOWL is poured, heaven's terse pronouncement is triumphant: "IT IS DONE!" (16:17).
The last plague of judgments -- the BOWLS -- soon will be poured out. Just prior to this happening, John sees and hears things that point to the Triumphant King, the One in control of ALL things. He hears victorious saints sing the glory Song of Moses and the Song of the Lamb.
Then John sees seven angels coming out of the temple in heaven. They are given seven golden BOWLS full of the wrath of God. The temple is filled with smoke from the glory of God and from His power.