Preaching to the Choir

Somebody used the phrase "Preaching to the Choir"

I know that it means somebody arguing a point to a group that already agrees with the point.

But why? It seems that there's no reason a choir is in agreement with the preacher. Maybe a bunch of them are there because they like singing, or the social aspects of the church.

It seems that it would be better used to mean arguing a point to somebody who has no choice in listening to it, and can't escape.