Raises the rate of wild Pokémon.
Illuminate is an ability that doubles the wild encounter rate when your lead Pokémon has it. and they are at least 5 levels lower, too. The opposite of a Repel effectively.
Illuminate makes it easier to find wild Pokémon while traveling, grass, cave, etc. While this ability is isolated only to four water Pokémon, it does not only affect encounters in water. You can bring it on grass.