Xenoblade Chronicles is as much a game about exploration as it is a game about adventure and combat- so it’s no surprise that you get rewards for exploring the world (or rather, for exploring Bionis and Mechonis). As you wander about the titanscape, you’re going to find yourself gaining two different sets of rewards for exploration.
The basic exploration uncovers Areas. Every region, whether it is a city, the area around a city, or a particular portion of land, is broken up into several Areas. You will gain experience for wandering into each area, which the game will note with a small pop-up message on the right side of the screen. These rewards are usually fairly small, but if you put them all together they’re pretty significant, so make sure you wander all over the place. Besides, how else are you going to find everything?
The other things you find when you explore are Landmarks. Landmarks serve several purposes. First of all, well, they’re landmarks. Most of them are fairly big and easy to see from a distance, so they can help you navigate (especially if you’re using the Rotate option for your Mini-map). Second of all, they note important locations- the Ether Light is the center of Colony 9's market district, for example, and most of the people who wander the markets can be found near there at some time of day. Third, Landmarks are your way of saving your butt when it gets handed to you.
In the developing tradition of RPGs, Xenoblade Chronicles is essentially an ‘unlosable’ game. When you lose a fight, rather than flat-out failing your quest, you will instead automatically teleport back to the last Landmark you visited and your party will heal up completely. While this seems convenient initially, it can cause you some aggravation- if you haven’t been exploring and locating all the Landmarks, you may find yourself ‘respawning’ a good ways off from where you got beaten, forcing you to spend some time making your way back there.
While it’s usually not as much as a few good battles, exploring will net you experience points and between that and Landmarks, it’s important to see all there is to see of your environment- so go out there and have a look.