Provides an unstable surface to exercise on, requiring you to use more muscles to stay balanced, providing a more complete workout. Core training is all the rage right now in fitness, not that is has not always been important but it seems the awareness of balance training with a strong stabilizer focus is popular at this time. Capitalize on that trend by adding not just balance cushions but the board as well. Muscle confusion keeps the body guessing.
If you want to target those deep, hard to reach stabilizer muscles you need to exercise on an unstable surface which forces your body to utilize those deep and middle layer stabilizer muscles which are typically not engaged when performing standard exercises.
Balance boards, wobble boards, balance cushions, balance pads, balance discs and stability balls are all tools to aid in balance training.