One reason was to create exspheres - powerful crystals which when equipped, allow the body to achieve feats far beyond ordinary limits, whether it enhances basic reflexes, allows for more movement, or surviving long falls, or even enhancing the artes that one may possess.

Another simply allowed a release of frustration. The Desians were told that it was their right to mistreat humans due to the prejudice they faced at the hands of them, and they took this persecution to high levels. The flourishing world, on the other hand, was free of the Desian organization, and ordinary half-elves were at the bottom of the caste system. In other words, it's just another cycle of destruction.