The ETC Source Four Full Top Hat serves two functions:  To control light spill, or to reduce the viewing angle to the lens. Sometimes poor fixture optics or poorly maintained lenses can cause light to be directed into the audience or onto scenery, and top hats can help control the light spill.  It is also often objectionable to the audience or camera to see the bright source of the light and top hats can help shield the bright source from view.