Board Thread:General Mod Discussion/@comment-26322120-20151231185738/@comment-27723099-20160114121436

I found another good explanation:

To Iluvatar, the world is like a symphony. The clash between good and evil gives meaning and enriches that symphony. The world isn’t meant to be a place where everything goes in only one direction; it would have no meaning. That is why good and evil are so necessary to the fulfilment the Song.