According to Hatha Yoga:
Hathasya prathamangatvadasanam poorvamuchate.
Kuryatadasanam sthairyamarogyam changalaghavam
Hatha Yoga Pradipika 1:17
Meaning: Asana is spoken of as the first part of hatha yoga. Having practiced asana, one attains steadiness of body and mind, freedom from disease and lightness of the limbs.
Asana is the first step of yoga practices as explained in the Hatha Yoga Pradipika by Swami Svatmarama. In hatha yoga nadis (energy channels) are an important focal point. Yogasana practices are used to restructure and purify these energy channels. Prana, the life force, flows through these energy channels. The pranic flow is obstructed when there are blockages in the nadis. With regular yoga practice, pranic flow is activated and redirected to remove the blockages. When prana flows freely, the toxins are removed and the body becomes supple and light. The practitioner is now prepared for the advanced practices of yoga.
According to Raja Yoga:
In the Yoga Sutras of Patanjali there is a concise definition of yogasana:
Sthiram sukham aasanam
Meaning: That position which is steady, still and comfortable.
According to sage Patanjali the goal of practicing asanas is to enable a practitioner to sit comfortably in a position for extended periods of time and practice the higher yogic sadhanas of pranayama and meditation. For this reason, in the eight fold yoga path of sage Patanjali, yogasanas come before pranayama and dharana.