The Moon increase the mob density at oskayaat considerably (they drop moon pin as an addition during the event), it also growths an special fauna (Moonlight Ipomea's stamen), allow the mobs to curse us and bring 2 bosses Tamru (who isn't available as an npc until the moon phase ends) and ayana who was the main character of the teaser story.
The Moonlight Ipomea's Stamen is useful with or without Moon to obtain rewards, but is only obtainable once during Moon phase, There's not any other widely known effect of the moon in Oskayaat so far.