The purpose of asana is as varied as the individuals who practice them. Some people wish to improve their health, increase their flexibility and strength etc. Others are looking for a way to obtain greater peace, relaxation and harmony in their lives. Some practice to increase their awareness, focus and concentration. Still others use it as their initial preparation to a greater understanding of truth and reality, and ultimately transcendence of the limited self.
All these different reasons have their value and place in asana practice, but it is also important to recognize the purpose(s) for which the practices were originally designed. Depending on one's chosen path in Yoga, the emphasis on asana may vary -
Raja Yoga – Asana is practiced to develop the ability to be able to sit comfortably in one position for an extended period of time.
Hatha Yoga – Uses specific positions to open the energy channels and psychic centres. In this tradition, asana are tools to higher awareness and provide the stable foundation necessary for exploration of body, breath, mind and beyond.
Jnana Yoga – Asana is used to increase concentration and purify the body, so that the mind is free to trace the 'I' thought back to it's source.
Karma Yoga – Poses are practiced to increase one's coordination and balance making action more efficient and harmonious.
Other paths – Whilst asana may not be directly specified as a part of other paths, this is not to say that they don't have value when combined with these approaches. Many of the side benefits from asana greatly contribute to preparing the mind-body for other practices.