Yoga is just not about asanas or some quick anecdotes to cure a disease or pain. It is a science of techniques, techniques to live a life to attain the ultimate truth. One of the techniques is the Eight Fold Path. It is a step by step discipline to attain wellness.
The Eight Fold Path consists of the Yamas: social restraints, Niyamas: observances, Asanas: postures to discipline the body, Pranayamas: control of bio energy through breath, Pratyahara: management of the senses, Dharna: concentration, Dhyana: meditation and Samadhi: trance or the ultimate truth. This is the complete study of Yoga, the union of mind, body and soul.
Yamas and Niyamas works on our attitude, behavior and our habits. Asanas and Pranayamshelps us to physically train ourselves. Pratyahara makes the mind the master that controls our five senses and Dharna, Dhyana and Samadhi is the state where the mind is balanced, in harmony and has the highest awareness.
So yes we need to physically work on ourselves to attain good health. Mind needs to be worked upon too. Thus yoga aims at both physical as well as mental health for wellness in life.