What is a Song of Moses Doing in Revelation? | Revelation 15

Chapter 15 of Revelation is a little short chapter, but in here we have the second song taking place in Heaven. Is the worship we see here liturgical or spontaneous? Could it be both? And in Moses song, there is a theme of justice which uncannily connects to the other song of Moses in the Bible found in Deuteronomy 32, where it says God will judge the land. After this we are introduced to the last set of 7 judgements - the 7 angels with 7 bowls.