Because of the instability of the ball, performing exercises on the ball requires more muscle to contract and be utilized by your body for balance. EMG studies show that doing crunches on a stability ball uses 30% more abdominal muscle than doing crunches on the floor. I guarantee that if you do a few sets of crunches on the ball with proper form (if you don't know ask a PT
), you will be more sore the following day than you ever have been doing crunches on the floor.
Think about it, the principal behind the use of a stability ball is basicly the same reason you prefer DBs and BBs over machines whenever possible. Its harder and more muscle is used. The end results may be different, but the principals are the same. Not only that, the stability ball is very versatile.