The reason why we sing is that, in Col 3:16 and Eph 5:19, the Lord told us to sing. God’s creation started with singing. In Job 38:4-7 the morning stars sang together. The nation of Israel sang their great song of deliverance in Ex 15 after God drowned Pharaoh and his army in the Red Sea. The Psalms are songs, many of which were set to music as in Ps 18. Jesus and his disciples sang a hymn together before Jesus went to the Mount of Olives and the garden of Gethsemane, before his death, Matt 26:30.
In heaven we will be singing praises to the Lord together. In Rev 5:8-9 John saw the church age saints singing before the throne. And in Rev 15:2-3 he saw the Tribulation saints singing before the throne. Then, at the second coming of Jesus, the armies executing God’s vengeance and taking kings captive will be singing together, Ps 149:1-9. Finally, the mountains and hills will be singing in the millennial reign of Jesus, Is 55:12-13.
If the full spectrum of time since creation involves congregational singing, you can be sure that we should be singing together in church. The Lord commands us to do this. Paul told us to sing psalms, hymns, and spiritual songs. Psalms are obviously the ones you find in your Bible. You can find many of these set to music in the Psalter. We can find hymns in our hymnbooks. And spiritual songs are songs that minister to your spirit rather than your flesh. They are the songs that honor God and drive away evil spirits, like David playing the harp to refresh Saul when the evil spirit bothered him.
When we sing, the word of Christ dwells in us richly. Many of us know Matt 6:33 from the song “Seek ye first the kingdom of God”. I can still remember Ps 118:24 from the song, “This is the day which the Lord hath made”. Many hymns combine the words of God in their stanzas and choruses and teach us the word of Christ. A good example is Great is Thy Faithfulness. The lyrics are principally from Lam 3:22-23. There is no shadow of turning with thee is from Jas 1:17 As thou hast been thou forever wilt be is from Heb 13:8.
We know that when we sing together from our hearts to the Lord, we are doing something that pleases him. Singing to the Lord is one of the things we can all do as a church together in unity. When we sing together, we praise the Lord and teach and admonish each other. For more information on this subject, see the Sunday school lesson The Word of Christ and Singing.