Here are some of the reasons that come to mind...

Don't you think these are pretty good reasons to do it?
Let me share my take on each one...

Release Stress

Because the first principle of Tai Chi is to relax, you will learn to release tension from your body. In order to release tension from your body, you need to release tension from your mind. By bringing your attention to your body and relaxing, you get to release your stress. Usually stress stems from the gap between where you are and the future/expectation. Tai Chi practice is about being in the moment. By keep bringing your attention to the present, you release the unwanted anxiety from your body.

Clear Focus

When you are constantly distracted from outside stimuli, it's impossible to have clear focus. In Tai Chi, you are to focus in on yourself. You may observe your surroundings, but you are not going to constantly chase whatever is going on around you. By finding your center and settling in to the core, you will see things clearly and be able to have a relaxed focus. Once you master this, you can apply this clear focus in your life to increase productivity.

Balanced Body

By being aware of your center and feel secure in your body, you will gain balance. Tai Chi is a constantly flowing movement and your center is shifting constantly. You will learn to recognize your center through practice. Also if you have weaker spot in your body, body tend to compensate for the weakness by making other parts of the body stronger. This creates the imbalance in the physical structure. Tai Chi awareness will encourage you to flow energy evenly throughout the body so you can develop the weaker part of the body as well.

Youthful Energy

When each cell get enough oxygen and each cell release toxin, your blood flows easily, muscles are warm, all the channels in your body are activated. It's like walking into the house after the spring cleaning. When your body is like this, and mind free of clutter, natural youthful energy will bubble up from within you. You can't stop smiling, because physically and emotionally you just feel so good. Keep moving with Tai Chi and access your youthful vigor! Legend says "if you meditate and do tai chi 100 days in a row, you gain back eight years of youth".

Longevity

Being fluid and being supple is one of the main focus when you get started on Tai Chi. The aging process is accelerated by being stiff. During the practice, you will train those stiffness out of your system. We are constantly reminded to be like a baby in the class. By constantly brining those baby energy in our system, perhaps we are reversing the process of aging. Most of all, I believe it's not about how long you live, but it's about how well you live when you are alive. Tai Chi will definitely assist you in the process.

Peaceful Mind

The mind set of doing Tai Chi is like being in the center of the storm. Eye of the storm is calm and peaceful. In this place, you have a sense of your truth and completely present with what is going on without being part of the storm. Being in peaceful state on top of the mountain by yourself is easy - in Tai Chi, we cultivate your mind so that even in busy modern life in the city, you can still keep your peaceful state. With this total acceptance and awareness, comes deep sense of peace.

Lose/Gain Weight

Tai Chi exercises and drill will let you get in touch with your optimum body. As you learn to listen to your body, you will start to get in touch with the natural wellness state and start to take care of your body accordingly. If certain self-image or past conditioning are the cause of your weight problem, then Tai Chi practice will support you to look inward then release those from every cells of your body through movements and breath. If bad habits or laziness are the cause, then Tai Chi practice will be a perfect place to create new habits in your body and life.

Release Toxin

Pollution, stress, anxiety, and emotional baggage - all these things may be in your system as a "toxin". You can bring eat all the good things, and exercise as much as possible, but if you have not removed these toxin from your system, these toxins are eating you up from inside. Toxin or blockages in your body may be in your joint, muscles, cells, emotion, or in your general energetic body. Tai Chi will move the energy throughout your body and help you to clear out toxins from your system. At least the movements will start to soften the trapped historical junks in your body.

Increase Flexibility

In our daily lives, most people do not use all the possible motion in our body - so by practicing Tai Chi is a simple way to keep your whole body in check. Tai Chi movements use full range of motion. (Arm and shoulder rotation, chest opening, knee bends, etc..). The places we do not use often become stiff like a rock. With gentle Tai Chi movements, you get to gradually loosen those stiffness. Stiffness in your body can stem from stiff mind. Tai Chi will gently challenge you to release attachments and belief, so you can live with flexible mind.