You've heard me talk core multiple times. All in good reason though. Your core is the powerhouse of your body. Whether you are a power athlete or a power mom. Having a strong core is beneficial for everyday functioning, (Think: lifting children, groceries, avoiding lower back pain, etc.)
This exercise is one that can be varied, so get creative. Balancing on the physio ball is something that can be intimidating at first glance, but it really just takes some guts. Once you start to get the feel for it you can branch out and try to increase your difficulty.
Start with just a normal knee balance. Find a wall or chair where you can stabilize yourself and take your time to slowly get on top of the ball. Once you feel like you've got your balance let go of the wall. First, just give yourself time goals. Stay up for 30 sec or a minute without falling off. Learn to use those core muscles and your inner thigh adductors to keep you in place. Once you've mastered that increase the difficulty by adding arm movement. Do arm circles out to your sides working those shoulder. Once that becomes too easy add weights. Do bicep curls or shoulder presses on the ball. You'll be surprised at how quickly you are able to increase your difficulty here. Heck, Before you know it you'll be standing on the ball!