A counterweight is crucial for magicians, needed in case an element threatens to overwhelm the magic user and draw him/her in. The mage would reach for the opposite element if they overextended themselves or are about to be drawn into the element. The opposing elements can also be used in battle, supposedly making them more effective against each other.
This also applies when trying to defeat elementals and the Devoured. In fact, the only way to kill an Devoured is to use a large amount of the opposite magic or a more powerful Devoured (also of the opposite magic). The only exception to this rule is Devoureds of chaos, who can be restored to their human state by one Devoured of the other four types of magic. There is no such thing as a Devoured of soul.
The concept and use of counterweight is included as the fifth principle in the "Five Principles of Magic": "All elements have a counterweight. Fire is the counterweight of water. Air is the counterweight of earth. The counterweight of chaos is the soul."