In a studio set up there is no need to worry about lighting being in the wrong place or being too bright because you have full control over it.
The problem arises when you're on location outside, and particularly if the sun is bright. It can be a photographer's knightmare. Harsh shadows on one side of the person and on the other side the light may be so bright it is in danger of losing the details in those areas in the photo.
This is where a reflector is a godsend! They can be used to reflect light and they can also be used to effectively diffuse bright light. On a recent model shoot in manchester city centre last week it was a necessary piece of kit.
Below are examples of shooting with the diffuser and without it. Notice how the girl's face has many bright and dark areas and her natural skin tone is lost on her neck and hand with the very bright sunlight.