Kids love collecting things, as anyone who's ever been embroiled in a Panini sticker-related dispute can testify. So it's no surprise the vast majority of videogames involve collecting things, whether that's coins or points or Pokemon or Enchanted Wizardweave Leggings of Maximum Fire Resistance.
Treasure World revolves entirely around the concept of collection. The twist is you don't pick up items as you explore the in-game environment - in fact, there's barely an in-game environment to explore. Instead, you gain new stuff by wandering around the real world with your DS in tow.
On hand to explain all this is a cheery professor type. He looks like what Dr Robotnik would have turned out like if he'd had a happier childhood. He's accompanied by the Wish Finder, a little robot character who is surely the lovechild of Clank and Nono (small robot you know). Their spaceship, Halley, has crashlanded in a moonlit garden, and naturally they need your help.