Preach to the choir

It’s a metaphor which basically means tell us something we don’t already know. Usually pertains to a conversation about something related to politics or religion but could mean anything . In church there’s always a preacher every Sunday. The choir boys and girls are the people who sing the melodies. So basically the choir has heard everything the preacher has to say every Sunday. Does newcomers would not have heard what the preacher had said. So when somebody says preach to the choir the metaphor actually means that the choir has already heard everything the preacher has to say. So if somebody’s rambling on and on about a subject that you already know about and agree with you could say well you’re preaching to the choir LOL get it?