Instances are probably mainly for farming material. That means getting hold of good equipment and then dismantling it.
Each instance has quests that you can only do once. You usually get these quests at a different level than the instance can be entered.
Example: The Litian Höhle can be entered from lvl 45, but you get the first quests at lvl 49.
You can find the quests in the quest search and more information here.