Ashtanga yoga is a wonderful system of yoga, employing a specialised breathing technique, the application of internal 'energy locks' called bandhas, flowing, synchronised movements called vinyasas, and fixed gazing points. It is a powerful practise both physically and internally- physically through the postures and vinyasas themselves and internally through the application of special breathing, locks and gazing points. The postures themselves also have deep internal effects on the person exerting themselves on the physical body and running through the nervous system into more subtle energetic aspects of the practitioner.
These days many peoples first exposure to Ashtanga yoga comes from doing classes in a gym where it seems that the main focus is on doing physical exercise or a workout. Whilst this is of course happening, as Ashtanga yoga will undoubtably work the body out very thoroughly and powerfully indeed, there is more subtle work going on that may not even be in the students awareness. Students may maintain an Ashtanga practise for years, feeling their bodies increase in strength and flexiblity and enjoying the vast health benefits but may be resistent or unconscious of the work going on below the physical level and may tell anyone who cares to listen that yoga is a 'workout' for them. However this wouldn't explain why they have maintained the practise for years and also there are far easier ways to workout the body than Ashtanga yoga! Other students will very soon get a feel of the more subtle factors going on in their yoga practise and generally these will very soon devote themselves to a more involved and frequent practise.
So back to the initial question , why do we practise yoga and what are the benefits?
Well it is probably an unsurpassed physical practise in that it works nearly every single muscle, tendon and ligament in the body. The postures also massage and stimulate all the organs and glands in the body and the spine is worked very thoroughly and deeply. The body is also given the opportunity to enter into inverted postures which have far reaching physiological effects. As the physical body becomes strengthened and purified this simultaneously starts to clear the way for more subtle energetic work. Concentration, focus and mood are all stabilised and the practitioner is able to work deeper and deeper into himself, layer after layer. Yoga is a practise that is deeply rooted and originates from our deepest most spiritual Self.