When you can't reach some note beacuse it's a bit high or too low, singig a harmony is a good way to solve the difficulty, and it sounds good too. And if everone sings the melody notes it gets a bit crowded in there, so it seems natural to find a harmony where there's a bit more space.

Actually what tends to happen a lot of the time in folk clubs and so forth isn't so much the kind of harmony singing where people are aimig for block chords, like barbershop singers, but rather people evolving different melody lines that come together at times and move apart. And part of that is because where people pick up songs orally (which includes listening to records), we tend to pick up and pass on variants, either because the variants were there in what we were listening to, or because our memories and are ears aren't all that accurate.