Postures, called asana in Sanskrit, along with breathwork, called pranayama in Sanskrit, are the basis for yoga.
While there are more complex or advanced practices within the entire spectrum of yoga, such as meditation, focus, and energy work, these two things are the fundamental "jumping-off point" for all yogic practice.
The postures, also sometimes called poses, serve several purposes. In the most ancient and traditional forms of yoga, the poses served to make the body stronger and more flexible so the yogi could sit for long periods of time in meditation. As time went on, the poses have taken on greater meaning, as more than just a vehicle to meditative sitting, in some of the major forms of yoga.
When T. Krishnamacharya (the founder of modern yoga, as we know it in the west) re-invented "yoga" in the early nineteen-hundreds, in order to interest Indian youths in the nearly extinct practice of yoga, he combined the ancient yoga practices, the philosophy of the Sutras (Patanjali) and the Hatha Yoga Pradipika (Swami Swatmarama), with calisthenics and military movements brought to India by the British, as well as native Indian militaristic training movements. The most distinct yoga school to emerge from this vigorous form, as Krishnamacharya taught to young, strong boys, is Ashtanga yoga, which Krishnamacharya taught to P. Jois. However, Krishnamacharya rarely taught in groups, and so each of his many students were given the type of training that suited them best. His son, TKV Desikachar most represents this style of teaching in his "Viniyoga", which is personalized and focused towards the needs of the individual, rather than trying to enforce one style on everyone, cookie-cutter style.
While we hear frequent criticism from some circles about how "westerners just focus on the poses and breath" in yoga, where else should we begin? Yes, the ancient yogis began with meditation, but they were coming to yoga to gain a spiritual state they referred to as "enlightenment". While some modern people may still be seeking this, most are not and are coming to yoga to feel better, either through healing some problem in mind or body, or to simply get in better shape or be healthier. Yoga is a wonderful tool to achieve both of these things, and it is the poses and breath that most facilitate this. Krishnamacharya understood that we were no longer ascetic yogis begging for our food and wandering India in search of enlightenment and he understood that yoga could be a most profound tool of healing for more average, regular people. I heartily concur!
So then, what is it, exactly, that the poses do? Well, there are several things. The most obvious is that they stretch the muscles, helping us to gain flexibility. The poses strengthen the muscles, as a great many of the poses are classic "resistance training", using the weight of the body against gravity. Next, the poses help us to gain balance and re-train the body, gaining more symmetry, alignment and postural balance. As well, the poses raise heat in the body, producing sweat, which cleanses the body systems and certain groups of poses have more subtle effects. For instance, the twisting poses are said to help cleanse the internal organs by squeezing bold out of them and allowing fresh blood to flood them, and the inversions are said to activate the lymphatic system, circulating the lymphatic fluids within the body and enhancing immuno-responses. It is my experience that the poses, when combined with intense breathwork, can enhance the endocrine function of the body, but the most important functions of the poses, I have saved for last.
These aspects, which I find to be of great importance, are perhaps more subtle than those I listed above. First, the poses create varying levels of stress in the body (as does any exercise or movement system), but unlike most regular exercise, yoga uses the breath to teach the student how to respond to the body-stress and shut down the stress-response. Through the long, slow, deep inhale and exhale, we learn how to over-ride the stress-response and become comfortable in the pose, and it is through this process that we grow, gain flexibility, strength and all the other wonderful benefits that the poses have to offer. Patanjali says in the Yoga Sutras that the poses should be comfortable and relaxed, and this is our goal. When we can hold a difficult or challenging pose by using the breath to shut down our physical stress-response, we are truly becoming yogis, and gaining one of the greatest benefits yoga has to offer.
For me, one of the reasons that the poses are so important is that they get us out of our heads and into our bodies. In the modern world, we tend to intellectualize everything, and thus our bodies are just a vehicle to carry our over-active heads around. The body is most often a frail inconvenience that occasionally provides a little pleasure. This tendency to over-intellectualize causes us to be perpetually stuck in our heads and strangers to our bodies, but the body is where we dwell, and it is only by getting into the body that we can open any of the other doors to higher levels of human potential and shut down much of the internal chatter. Some schools of thought, both psychological and shamanic, believe that the body is the subconscious, and thus, our stored or repressed traumas are stuck there until they are worked through in some manner. While any form of bodywork is excellent for moving these held energies, yoga is particularly powerful in this regard, because it gives us tools to do this ourselves, empowering us and allowing us to participate in our own recovery and healing.
Finally, in addition to all these potential benefits, everything that we learn on the mat can translate directly into our day-to-day lives. For instance, when we are in a difficult pose and our body enters its stress reaction, we calm ourselves through the breath. In the same way, in our lives off the mat, when we encounter situations, which produce a stress-response, we know how to calm that response from our practice of yoga poses. If, for instance, we must practice a pose over and over to really learn it, we gain patience, and holding a pose for any length of time, especially when it is uncomfortable and we have to turn to breath to get through it, teaches us patience on the mat, and we can then apply that off the mat in our day-to-day lives. So when we hear people say that yoga builds grace, poise, confidence, balance, patience, etc, it is not referring to some un-quantifiable, spiritual theory, but to taking what is learned in the practice of the poses and applying that in the daily existence. I found, in my own experience, that this happened quite on its own, without me having to think about it. It is simple. As we learn these things while practicing postures, they naturally translate over into other areas. So for all these many reasons, I count the poses as the beginning point for most beginning yoga students and I count the poses as one of the most important and fundamental tools in the entire "yogic toolbox".
