The outer ear is surely a shell to catch the sound waves more effectively. Therefore adding a cupped hand surely acts as an extension to that. In theory. Not only that, in most people the ear is a fixed object. The human hand, however can be angled to alter the tone volume from different directions. This is most effective when other instruments/singers are in close proximity, but also in a room sounds echo from different objects and walls and here it can also be a help. However, I can't see an advantage to singing solo in an open field in this way. In that case the cupped hand might be better round the mouth, or in some cases covering the mouth.