There is a vast difference between a song that you sing, or that I sing, and one that WE sing together. The difference is breathing. A communal breath has to be accounted for in accompanying a group singing, whether a hymn or a national anthem or a carol of any kind.
I have heard virtuoso musicians utterly miss this critical point, as if they know only how to breathe as they are used to playing: alone.
Actually, it is not as simple as that. The fine points of many compositions is the subtlety of phrase markings and articulations. I was not made aware of them in piano writing until adulthood. How it changes "ordinary" harmonies into inherently dramatic counterpoint, impossible to predict or repeat.