Inverse square law

Double the distance between a lamp and your face and the light landing on you drops to a quarter, which is two stops. Halve the distance and you win those two stops back. On a set the useful consequence is falloff rather than brightness. A lamp placed close has a steep gradient, so shifting your head 20cm changes the exposure visibly and the wall behind you goes dark. Pull the same lamp back two metres and you get a flatter, more forgiving pool of light you can move around in, with the background lit as well.