Allow me to explain.
The world map (or most of it) is "unknown territory" to our young hero, and it must be discovered through exploration. Not only will you uncover large pieces of land and new kingdoms and dungeons, but the more land you uncover, the better chances you'll have of finding extra items for your journey.
As in previous games, there are no random encounters on the world map, so you're free to explore at your leisure without fear of heavy battles every few steps.