Most grow lights are prone to generating "hot spots" directly under their lamps, heat shields have been developed to help reduce this problem by diffusing some of the heat and light off at an angle and out over a wider area.
In theory this will allow you get your light a little bit closer to the plants, and this should help with penetration to the lower parts of the plants that don't usually get much light.
Overall we quite like this product, if you haven't used them before it's worth buying one to see if it works with your lighting setup.
They are especially useful where height is an issue, and are recommend in grow tents where horizontal lamps are being used.