Sitting asanas (poses) are necessary for a firm foundation in Pranayama (breath control). They bring firmness of body and steadiness of mind. These poses train the body to sit firmly, extend the spine, bring quietness to the mind and composure to the body. Some sitting asanas, such as Virasana, extend and expand the chest so that the thoracic cavity is expanded for breath control.
These days everyone sits in chairs and have forgotten how to sit on the floor. Sitting poses re-train one is sitting on the floor, which reduces trouble in the knees, ankles, hip joints and spine.