The beam from a flash unit is cone-shaped. Photograph a relatively close object and most, if not all, of the light cone will cover it. But as you move the flash farther away, a smaller portion of the light cone hits the subject, as more of the light sprays wide of it. At a great enough distance, the flash’s light becomes imperceptible to the eye as well as to a sensor or film. Imagine trying to water a small plant at a great distance with a garden hose set to a wide spray—you may pump out gallons per minute, but very little of it will actually hit the plant.