What does "preaching to the choir" mean — and why is it funny?

informal

Meaning

Arguing in favour of something to people who already agree with you, so the effort is wasted.

Where it comes from

A twentieth-century American phrase; Britons often say 'preaching to the converted' instead.

Why it is funny

The gentle humor is the wasted earnestness of the picture. You imagine a preacher passionately trying to win over the one group in the building — the choir right behind him — who were never going to disagree.

Used in a sentence

"Telling me we should travel more is preaching to the choir."