A very helpful post from Dr. Piper:
In the Religious Affections Jonathan Edwards ventures this explanation of why there is song and poetry.
"And the duty of singing praises to God, seems to be appointed wholly to excite and express religious affections. No other reason can be assigned, why we should express ourselves to God in verse, rather than in prose, and do it with music, but only, that such is our nature and frame, that these things have a tendency to move our affections."
For this to have the weight it does for Edwards we need to remember that 1) “true religion consists very much in the affections,” and 2) there is no true Christian faith without the affections being awakened, and 3) God is most glorified when he is affecting us and not just known by us.