I do it because I am descended from a long line of petrol pumps.

Seriously, as everyone said, it is using bone conduction to help sing a harmony when singing in a small group.

Different when singing in a choir. No-one can tell in the larger number of singers if you momentarily wander off key. Choirs have their own idiosyncrasies, like dressing in uniforms, but having soloists in a slightly different strip. Thwarted militarism, I call it.