The ultimate purpose of yoga is to raise our level of consciousness, to allow us to be in control of our minds rather than our minds being in control of us, thus allowing us to see reality as it truly is rather than our own mind made realities.  Yoga is a lifestyle, not an exercise.  The physical benefits we receive from practicing yoga (of which there are many) are therefore the sideaffects rather than the goal.  Hatha yoga is a moving expression of yoga that takes the form of holding different asanas (poses) in a flow that allows us to coordinate our movements with our breath.