Preaching (versus Singing) to the Choir

0 I've heard both "preaching to the choir" and "singing to the choir" used to mean that something is unnecessary. That doesn't make a lot of sense with the former, though. Why would preaching to the choir-01del00or to anyone at church, for that matter02del00-be useless?02br
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00"Preaching to the converted" is similar, but seems to implies knock-on-door-preaching (whereas "choir" suggests pulpit-preaching). I've also never heard anyone say it ... (archaeic?)02br
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00In some limited googling, I found:02br
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00On fine spring Sunday mornings, attendence at church has been known to be very sparse.02br
00 Whatever the rest of the faithful are doing, the preacher and the choir have a reason to attend. In fact, the choir may be the preacher's only audience.02br
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00This suggests an entirely different meaning, but sounds the most logical to me.02br
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00Anyway, can someone provide more information? What's the meaning behind these?0-