I still don't understand much of it
So basically, missions create areas.
Eg. one mission creates a fire region, so you better prepare to fight fire monsters. Another mission creates an ice region, so you better prepare to fight ice monsters. If you combine both missions, you'll have an area with both fire and ice monsters, making it alot more difficult to prepare to fight them, and due to the added risk, the reward is increased? Or do you get the reward of both missions, and afterwards there's some NPC or player market or such where you can sell them and buy better stuff with the profits?