Throughout history, there has always been contention between these forces. Good triumphs over evil, chaos erupts where there was once law. These forces are necessary to the advance of civilization. Imagine a world where there is nothing but peace.
Law reigns supreme and there is no unhappiness or strife.
This world would be a boring place indeed. No strife to keep people motivated, no reason to advance in anything. Imagine a world run by evil and chaos. A dark lord rules over everything, people are enslaved, and civilaztion ceases to exist as we know it. A balance between these forces is necessary to keep our civilization running.