At this point some people say they thought Taichi was for health. Taijiquan has certainly been promoted for its health benefits, and as a gentle form of exercise that can be practised well into old age. Indeed it’s often offered to elderly people, along with yoga, as a safe form of exercise.
Nevertheless, Taijiquan was developed as a fighting skill. Performing the moves slowly and without force is a method of training, which allows relaxation and sensitivity to develop and emphasizes good posture. When these are developed, using force swiftly and unexpectedly can be used to beat a stronger and larger opponent. It is said that a Taiji expert can defeat anyone, seemingly without effort.
Qigong probably evolved out of the martial arts. Taiji encompasses methods that are specifically designed to strengthen and nurture health. They are adjuncts though, to the primary purpose of training, which is for fighting, and these days, for competition. There are no Qigong competitions but health and wisdom come with practice.