Ocassionally, one of the characters give you a call, and there will be a "swarm" of a pokemon. I'll use Buizel as an example. If the game says there is a swarm of Buizel on route 30, then if you go there, you can find more wild Buizel than usual. In most cases, the swarmed pokemon doesn't appear there at all, so it's an opportunity to catch new pokemon.