Reflector design can dramatically impact the amount of light reaching your plants. In the diagrams pictured (assuming they are all using a 1000W lamp) you can easily see how three different reflector designs can change the focus and intensity of the light.
Another way to think about this focusing effect is to compare it with adjusting the spray pattern of your garden hose. Without a hose head, the water does not have guidance on how to exit the hose. With a hose head, you can focus the water spray and shoot it out of the hose in a variety of streams or spreads.