Of course, yoga does far more than keep us limber. It releases tensions from our bodies and minds, allowing us to drop more deeply into meditation.  In yoga, "flexibility" is an attitude that invests and transforms the mind as well as the body. - from yoga journal

Joint lubrication, improved healing, better circulation, and enhanced mobility are some of the benefits derived from stretching.

The best type of flexibility combines improved range of motion with improved strength, creating a more useful flexibility.  According to David Sheer "If you only increase your passive flexibility without developing the strength to control it, you make yourself more vulnerable to a serious joint injury."

Gray Kraftsow, founder of the American Viniyoga Institute and one a highly respected yoga teacher, says "Alternating between contracting and stretching is what changes the muscle. Muscles relax and stretch further after contracting."  The breath is a useful tool here, helping us to stay in a pose and be a link between our consciousness and our autonomic nervous system. 

But any good teacher will also tell you that yoga isn't just about stretching. "Yoga is a discipline that teaches us new ways of experiencing the world," Judith Lasater, Ph.D. and physical therapist, explains, "so that we can give up the attachments to our suffering." According to Lasater, there are only two asanas: conscious or unconscious. In other words, what distinguishes a particular position as an asana is our focus, not simply the outer conformation of the body.

It's certainly possible to get so caught up in pursuit of physical perfection that we lose sight of the "goal" of asana practice—the state of samadhi. At the same time, though, exploring the limits of the body's flexibility can be a perfect vehicle for developing the one-pointed concentration needed for the "inner limbs" of classical yoga.