Self-defense is definitely a part of it. You step into the tall grass, and you'll be facing off with a whole slew of wild Pokémon, and some are more than happy to defend their territory.

[He snatches a Pokéball off his belt.]

But there's a few more incentives. Battles make Pokémon stronger, and if you use one of these, you can capture more Pokémon to add to your team. And trust me, you're gonna want at least six.