"Unless the singer has a hearing defect, I don't see that it's necessary,"
Lucky old you - I've sung and been interested unaccompanied singing most of my life and have always been acutely aware of the difficulty of maintaining pitch (usually a tendency to rise) without some sort of device to help; particularly necessary with unusual musical intervals.
We've recorded numerous descriptions of some of the singers in the first half of the 20th century sitting facing a wall or into a corner, with their backs to the listeners so they could experience the reverberations of their own voices. In rural Ireland, the singing was virtually all done at what they called country-house gatherings, ceilidhs or 'cuirds' - in other words, in the home.
Jim Carroll