As there is a opposite of everything, they are also needed. If there is Harmony, Chaos must live.

Ever since Discord's agreement to work with Celestia. The Chaos was uneasy, for its vessel had been quieted. It needed a way to wreck havoc. So it stored its energy, waiting for the time to intialize its new plan against the Elements of Harmony.

(This is my take on a new villian or whatever. First fic on FIM. May take long time to update...cause names are just such a block.)