Steps DDC brightness up or down for all found DDC displays. DDC brightness means the real physical back lighting.
While laptop screen brightness is already controled directly on most Desktop Environments using special BrightnessUp/Down keys, this might me useful to control an external HDMI screen brightness on a laptop. Or to control all 3 screens on DisplayPorts on a desktop system.
As BrightnessUp/Down are often a combination of Fn+F3 and Fn+F4, it's useful to have similar shortcuts like Super+Alt+F3 and Super+Alt+F4