Asana is considered to be a practice of Raja Yoga, but asana actually underlies all types of Yoga. To practice Yoga you must be incarnated—meaning living in a physical body. The bhakta chants with her heart and voice, which are part of her body; the jñani sits and meditates while in a body; and the karma yogi certainly performs his compassionate actions by means of his body. 
Asana pertains to our relationship with the Earth and all others, and our means of relating to others is physical. The practice of asana can bring us directly to enlightenment because the only thing that stands between us and enlightenment is our perception of ourselves and others. The karmas generated by our interactions with others are stored in the tissues of our bodies—in fact, our karmas actually make up our bodies—so moving the body through asana practice has the effect of purifying our karmas, helping us feel more comfortable in our bodies and in our relationships with others, and ultimately leads to freedom and liberation. Asana practice has the power to sharpen our senses and restore us to our natural state—the state of unity with all of existence, the state of eternal joy.
