As they exist in most systems, where they just are on at night, they are more than anything else asthetic. Any benefit they serve is negligable.
However, corals do spawn/reproduce in response to lunar cycles. I am not sure if this is because of the lighting levels produced by the moon or something else though. Whatever the case, the moon is never just "on" or "off," the amount of light that hits the ocean is in response to the lunar cycle. Standard moonlight systems cannot reproduce this, but I don't know if it would have any effect anyway.
To find out, I'm working on a moonlight system that varies the lighting level of the moonlights to cooincide with lunar cycles. If it works, fine. If not, it's still cool to brag about