Revelation is a book with many songs of praise to God for who He is and what He has done. Rev 15:2-4 has another such song (this time accompanied by harps) called ‘the song of God’s servant Moses and of the Lamb.’
The songs associated with Moses (Exodus 15 & Deuteronomy 32) focus on God’s power in delivering His people from Egyptian slavery and His faithfulness to His people. The song of the Lamb focuses on what Jesus has accomplished for us through His sacrificial death (Rev 5:9-10). Both songs acknowledge who God is (the Rock, the Lord) and this song does too, acknowledging God as ‘King of the nations’, ‘Lord God Almighty’ and the One who is holy before pondering how His deeds are great and marvellous, His ways are just and true and His righteous acts have now been revealed.
All our songs of praise should reflect this dual emphasis on who God is and what He has done, because we are so prone to forget these truths. This is the yardstick for what we sing in our services and what we sing in our personal praise times with the Lord. It matters what we sing; it matters that we sing truth. There is a place for our ‘response’ in songs, but we can only respond rightly when we know who God is and what He has done for us.