I thought I'd take this full moon day, a day of rest from our ashtangic labors, to ponder and hopefully say a couple of useful things about this question. A deceptively simple question, with a deceptively simple "official" answer. I believe the "official" answer would be something along the lines of, "We practice so we can bring out our highest, most self-realized being."
But what is my highest, most self-realized being? Is this something I attain when I am finally able to go into, say, kapotasana without the slightest bit of physical effort or mental anxiety? Or does it come when I am finally able to do viranchyasana? A moment's thought will tell us that the answer to these questions has to be "no." For one, I might never be able to do Viranchyasana in this lifetime. Even if I am able to, I might get to a point where old age and/or changes in my physical condition make me no longer able to perform that particular posture. Then what? Does that mean that at that point, I lose my most self-realized being?
I can go on and on. But I think you see where I am going: Despite the fact that we spend a lot of time on our asana practice, one's asana achievements probably has little (if anything) to do with how self-realized we are in the final analysis. So, the same question stares us in the face: Why do we practice? Or, more precisely, why does our practice take the precise form it does (i.e. asana), when the condition of this practice has little or nothing to do with the condition of our self-realization? If anything, the asana practice presents numerous pitfalls into which we can easily stumble, and which can derail us from the spiritual path. Narcissism ("look at me in such-and-such asana"), frustration and obsession over injuries and/or postures we cannot do, obsession over how we look (both on and off the mat), obsession over what we put or do not put into our bodies. The list goes on and on. It is too easy to (mis)identify these things with the practice itself while mouthing that official line and deluding ourselves into thinking that we are becoming more self-realized. Or maybe I'm being too harsh: Maybe the truth is that we are becoming more self-realized, even in the midst of our obsessions. Maybe the obsessions are phases or stuff we need to get through in order to understand who we really are (as opposed to what we happen to be going through at particular points in life/practice).
Here's my take on this question. Perhaps self-realization, whatever it is, is not something that hits you as an epiphanic bolt of lightning while you are in, say, padmasana one day during your practice. Perhaps it involves more of a deep understanding of our place in the order of things, and occupying that place and playing a certain role in that order with deep contentment. I remember this thing that I read many years ago, when I picked up B.K.S. Iyengar's Light on Yoga for the first time. I am paraphrasing here (I don't have a copy of Light on Yoga on me right now), but I think Mr. Iyengar said something to the effect that the purpose of the asana practice is to strengthen and purify the body so that it becomes a powerful vessel in the service of the Lord. Even though I do not believe in a personalized deity, this line really struck me when I first read it. In my personal opinion, I feel that if one sees the Lord as being inherent in every living being, then we can take Mr. Iyengar's injunction here as a reminder that practice is something that we do so that we can serve our fellow living beings better. Don't quote me on this, but I think Guruji would agree. In a recent blog post, David Robson has this to say about the practice:
'Ashtanga Yoga is “householder yoga.” It is a practice for those with family connections, and social duties, not a practice for monks or renunciates. For Ashtangis, our practice on the mat is a means to help us live better lives off the mat. We limit the formal practice to two hours or so each morning so that we can meet our obligations in the world. Our healthy bodies are only vehicles to help us along this path. In reference to asana practice without higher intentions, Guruji wrote, “It would be a shame to lose the precious jewel of liberation in the mud of ignorant body building.”'
'The Bhagavad Gita states, “One who outwardly performs his social duties but inwardly stays free is a yogi.” We cannot practice detachment by avoiding life. If we haven’t made any real connections, what is there to detach from? Healthy relationships require a lot of work. If we can devote ourselves wholly to the work, without attachment to outcomes, we manifest our higher nature in the service of others.'
So perhaps liberation or self-realization can only emerge amidst our "bondage" to our daily responsibilities. The Indian poet Rabindranath Tagore says, "Emancipation from the bondage of the soil is no freedom for the tree." Similarly, from the perspective of the practice, true freedom or self-realization can only emerge when we are firmly rooted in the "soil" of our daily responsibilities and connections.
Happy Moon Day!