Its the 3rd limb in the 8 limbs of Ashtanga yoga, but we use it as an adopted first limb :). Using our physical body is the most tangible and practical way to start working on oneself.The body is what we know, what we can feel.
Asana is meant to free up the body; through practice we create flexibility and as the body becomes more moldable and pliable...flexible, our minds begin to mirror that. Our thought process and awareness changes, which really just clears up our vision of things. Practice creates heat, which serves to burn all the residues of stuff we no longer need, clearing our vision and the way we perceive reality. For me asana teaches me self acceptance:
I used to not like what I saw in the mirror, when I dared to look; however through the practice of ashtanga yoga the way I perceive my self has drastically changed. I am now aware that my body is this beautiful flowing feminine container that holds my soul, my light, who I am on the inside, which really is what I see when I look in the mirror now. It is an every day journey, and it's not always so easy to remember that, but that is why I practice. Every day I plant the seed to self acceptance and love, and I do my best to understand that what's inside is what matters and that's what needs to be fed.
Once this self love is established then we can understand the importance of yama, the way we relate to who and what's around us; and niyama, the way we should treat ourselves to be whole.
It's a very natural shift, we just become more sensitive and aware of the ourselves and therefore of what surrounds us, we want to take care of both so we may be happy.
Because I learned, and am learning every day, to care for myself I can begin caring for whats around me, and commit to that.
So asana is the doorway, its just the leak of the iceberg its not an end in itself.. We dive in and we discover the inner workings of who we are...we become curious.. we shift our awareness.