The central aim and the highest purpose of Taiji (Tai Chi) practice is internal growth.
Other benefits include a healthier and looser body, greater energy flow, a strengthened immune system, some self defence ability, increased awareness and a quieter state of mind. For some, these benefits may be a large part of what initially draws them to Taiji. But in time, as our understanding deepens, the real purpose of the training - using the body to train the mind - comes more into focus, whereupon the mind to body coordination becomes more important than the body itself.
The essence of Taiji is thousands of years old. It has been refined by long chains of highly evolved old masters over hundreds of years, and the benefits verified by hundreds of thousands of practitioners.