In two words: experience points. Heaps and heaps of experience points.
Pokémon games aren’t known for being challenging. That doesn’t mean you can’t spice things up.
Just because you beat the Elite Four and become the champion of Hoenn doesn’t mean Pokémon Omega Ruby and Alpha Sapphire has nothing more to offer. If you keep playing after you beat the game, you’ll have a chance to experience something wild.