LED and Moonlights are most commonly used for a light at night to observe your organisms during the night. These small Light Emitting Diodes are not powerful enough to disturb your corals and fish but are powerful enough to help you see whats going on at night. These lights will also encourage spawning habits in many marine animals. LED lamps are a great way to catch those pests that come out only at night to eat your corals, with LEDs you can see them swimming and hopefully catch them before they do any damage.
LEDs are not mandatory but are a nice accessory to any aquarium for the occasional after dark viewing.