If you stay leaving on a chair, you'll lack back muscle and you'll likely have back pain, leading to worse posture, leading to even less muscle, etc. A yoga ball forces you to muscle your back.

Along with the general back muscling, it will develop small "stabilizer" muscles that frequently don't get enough activation. This can actually help make you less prone to certain types of injuries.

It's much more active than just a chair and allows you to move more.

Just throw the ball to your next meeting. No more tiny wheels sound.

Unlike standing, you can rest on a Yoga ball.