Taichi is an accessible practice and some physical and mental health benefits of practicing Taichi have been proven by Western scientific methods; others less so but are thought of as likely.
The NHS believes that there is scope for more rigorous research into the health benefits of Taichi, but promotes it as a healthy exercise, especially for older people. People also report spiritual benefits, which are more subjective, too.

Taichi is a practice that is accessible to everybody:
it is low impact and the less martial styles put little stress on bones, joints and muscles.
It can be practiced anywhere with very little space - three square feet is plenty. People say it can be practiced in a lift. It can be practiced indoors or out.
It requires no special clothing or equipment.
It is done at your own pace.
The risk of injury is low.
It is ideal for people with medical issues like arthritis or recovering from injury as the movements are gentle and low-impact, and practice is at the pace of the practitioner.

Because Taichi is a mind-body practice, the benefits relate to both mind and body.

Mindfulness- a focus on a single point or on being mindful of what one is doing - can contribute to healing, too. American studies on a mindfulness-based stress relief program for people with chronic disease found that the patients showed a significant decrease in chronic care episodes. Taichi has been called “moving meditation” and as such, can contribute to physical healing and there is some western scientific evidence and much anecdotal evidence to suggest the benefits of Taichi.
  
physical benefits

improve balance. Improved balance, apart from being a good thing in itself, is of particular benefit to older people, as it will reduce the likelihood of falls, which are especially dangerous for older people where falls and broken bones can lead to serious illness. Taichi can improve balance by training the proprioception in the nerves and muscles of the lower legs. Proprioception is the body's ability to sense the movement of limbs and helps us understand where legs and arms are without needing to look. Proprioception makes us less wobbly and less likely to lose balance.

Improve joint mobility. Improved joint mobility is important for anybody but especially important in people suffering from arthritis. Research has suggested that Taichi can improve ankle, knee and hip mobility of people with rheumatoid arthritis.
improve flexibility. As people progress, they find that they can kick higher, squat lower and wider and generally become more flexible.

lower blood pressure. Some studies have shown that practicing Taichi can lower blood pressure.

Lower cholesterol: some studies have shown that Taichi can reduce “bad” cholesterol levels.

Aerobic fitness: Because non-martial Taichi is a slow-moving activity, it does not in general raise the heart rate, but some postures are challenging to the muscles and this makes lungs and heart work harder.

muscle strength: practicing Taichi with bent knees, in more of a squatting position, works the leg muscles. Slow-moving arm movements also work the shoulders and back muscles.

Taichi is a recommended by some health practitioners as a practice for people with certain chronic diseases. Patients suffering from heart disease, asthma and other breathing disorders and joint issues such as arthritis are thought to benefit from the slow, gentle movements and mobility required.