Stress is a killer. It can also make you fat. There's an interesting article on a website that outlines the chemical connection between stress and poor health. Read it by clicking this link.
One of the main reasons to do chi kung is for stress relief. Most of us don't train our bodies and minds to relax. From the moment we wake up, we're on the go.
Through chi kung, you teach yourself to react to stress by calming the mind and body. It takes practice, like any other skill. Eventually, with practice, you will achieve a more relaxed state of mind. At that point, the health benefits will already start to accrue, even if you haven't yet made the connection between your new ability to handle stress and your improved health.