Ah, good eye! The event CE's increase the spawn rates of specific enemy types, like in the Garden of Sinners event. Each base spawn of an enemy has an additional chance to spawn another one of its kind based on the CE's you bring. For example, with one CE each enemy will have a 25% chance of spawning another of its kind. This is capped at 100% spawn rate (4 separate CE's, or one Max Limit Broken CE), guaranteeing double the enemy-type spawns. More monsters also increases the total monster materials drop chance, too!