From a yogi's view, asanas are more to gain the will from mastering your body. If you have properly achieved an asana, sitting in it properly, without moving, or your muscles fidgiting etc for 3 hours, then it is usually considered "mastered." The reason to master an asana, is because your body becomes accustomed to it, the body will no longer be a problem when meditating, and the first obstacle to meditation is that of your body, itching, fidgiting etc, such is why there are asanas. Most things in hatha yoga (asanas, pranayama) are used to help achieve raja yoga better.
One can go into it more though, the asana should preferably have a firm base (thus why you have padmasana, with your legs wrapped etc) because it is then considered that you can correctly and deeply meditate. Just as you should not expect to build a house without firm foundations, so you should supposedly not expect to meditate well, without an asana with a firm base.
The other reason is that the back should be straight. This goes into both physical and energetic concerns. Firstly, considering in deeper meditaiton you are subject to suggestion to a degree, you do not want to be slouching, lest you take it on as proper, how much better it is to sink into your mind that a straight spine is preferable. Not on ly that, but a slouching spine does after time, get uncomfortable, but then, so does a straight spine! Considering that the spine is also the main area of the central nervous system, which meditation seems to affect alot, you want it to be in it's best condition, so to speak.
To look at a specific example, my favoured asana is known as siddhasana, considered by many to be second to padmasana, and by a few, to be superior to it. The base is extended and firm, and the spine is 'straight', the left heel presses on the anus and PC area simulataneously, and the right heel at the base of the sex organs, ankles crossing directly over one another. Energetically if you look at that, one can notice that the heel closes the anus, an area associated with excretion, not only of faeces but of energy as well. Blocking this, is meant to prevent that from releasing, to add effect to this, the muladhara is also pressed and usually slightly tensed (pc muscle area) this prevents energy from leaving downwards, and any energy that is drawn from the atmosphere, through the feet, can also not escape, due to the tension, it is released upwards, through the spine and the chakras above it. That is why siddhasana has it's name, it was said to, when mastered, give the person many siddhi's. It is often associated with helping to awaken kundalini, and useful to those engaging in some form of celibacy, due to the sexual energies being sen tupwards, for more spiritual endeavours.
So yes, in a yogic thought, asanas are important to a fair degree, but then, it is more the control of the mind that is above the body. If you can achieve longer meditations without using an asana, then you should perhaps do it.
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*~*We have eyes, but do we truly see?*~*
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