Without meaning to give offense to choir members (I sure as heck am one), I suspect that it's because a lot of choral members aren't very good readers, so they need to see the scoring of parts on pages to see words, not as much as notes, to get their complicated entrances right.
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It's also got a lot to do with voice versus instrument. If you're playing the violin, then you know you count 8 bars of rest and then put your fingers in a certain position on the frets, draw the bow across the strings and you've got a C (for example).
As a singer, counting the 8 bars of rest is not an issue but, unless you have perfect pitch, coming in on the C can be. However, if you know that alto's are singing a C just before you, or that the violins play one three notes before you come in then you can pitch the note much more easily.
OP
