Brightness is the single most important quality of a light source. The more light there is, the smaller the aperture or the faster the shutter can be used when taking a photograph. With bright light one can always use the camera settings to lower the level of light intended to act on the film. When there is not enough light, there's no setting in the camera that can solve that deficiency. This parameter is very important even when we intend to use the same opening or speed: the brighter the light is, the slower can be the film we use for the same scene; slower films have finer grained film. The finer the grain, the higher the definition of the image (the bigger can be the enlargements of our slides).