“Praise the LORD! For it is good to sing praises to our God; for he is gracious, and a song of praise is seemly.” (Ps 147)
Why have a choir? It seems to me that there are several important reasons why it is good to have a choir:
1. The Church Fathers believed that to sing a prayer was to pray twice. This was simply because singing involves the whole body; it isn't purely a mental act. It is therefore appropriate to sing in Christian worship because we worship the Word made flesh - we are called to worship with our bodies. Singing a prayer is therefore a more fully Christian form of worship than simply saying.
2. A choir can function in a way that enables the wider congregation to sing themselves, either by supporting the wider congregation in what they are singing or by expressing something on their behalf.
3. Some elements of worship are best sung by specialists - this has always been the case, as can be seen by the practice of Temple worship in Old Testament times, and by looking at the Psalms.
4. The corollary of this is that some people have the vocation from God to be such specialists - God has called them to offer up their particular talents in this form, and without the possibility of that expression they are prevented from being fully human.
5. Ultimately, the point about singing in worship is that this is what the angels do, and the purpose of our singing in worship is to share with the singing of the angels, to be taken up and transformed by that beauty.
In other words, if we accept that the chief end of humanity is to glorify God and enjoy him forever, singing is an essential part of this.