As a contemplative practice, tai chi can help increase concentration and reduce stress. I have had students improve their COPD symptoms from doing simple qigong exercises, and others with PTSD and anxiety disorders find relief from tai chi and qigong. And I have worked with people with Parkinson's Disease, and they have found it beneficial.
Tai chi won't help with weight loss, and it is not a substitute either for cardio exercise or resistance/weight training.