With every event, there are new rewards. Most of the time there are lots of primogems to be collected. Other than primogems there are limited rewards that you can only get from completing the event. Most of these limited rewards are purely cosmetic. The other rewards that can be collected as rewards from events are character leveling material, otherwise known as character XP, and weapon leveling materials, weapon XP in other words, and mora, the currency of Genshin.