When it comes to catching wild pokes and getting items, what you do is spend "Watts" gained from walking to get a chance to do so. Otherwise, it's just your little poke runnin' along. "Watts" are also used to unlock special routes in your copy of HG/SS, so it's advantageous to use the Pokewalker. And if your lucky, your poke will level at the end of the walk.
-- Info from Nintendo Power (summarized by Shards)
Totally useful for me. I walk to/from school everyday, and often spend the afternoons walking about, going to Starbucks and the ice cream place down the street, LOL. And I have far-apart classes, and a timed mile two weeks after HG/SS comes out. LOL. Great timing, Gym teacher!