Why is flexibility exercise important?

The purpose of flexibility exercise is to create greater suppleness in the muscle tissue.  Why is it important to have supple muscles?  There are various reasons, with the two most important, in my opinion, being injury prevention and increased muscle development.  We generally refer to flexibility exercise as "stretching".  People sometimes argue about whether it is more important to stretch before or after an exercise or training session.  Honestly, this argument completely misses the point.  The discussion is rather about what type of stretching we do prior to a bout of exercise versus what type of stretching we do following it.  There are a few different types of stretching but I will discuss two of them here.  The first is dynamic stretching and the second is static stretching.  Dynamic stretching is appropriate prior to exercise.  The term "dynamic" connotes movement and this is what we do in dynamic stretching.  We move the limbs through quick and powerful movements that warm up the muscle tissue and prepare it for exercise.  We do this mainly for injury prevention.  A few examples of dynamic stretching exercises include high knees, the A skip, bounds and carioka.

At the end of an exercise session we engage in static stretching, which is the type of stretching most people are familiar with.  In static stretching, you move the muscle into an elongated, or "stretched", state and hold it in that position for 10 to 15 seconds, generally.  The reason we do static stretching at the end of our session is because it provides a mechanism by which we can work the lactic acid build up, from exercise, out of our muscle tissue.  By doing this, we decrease subsequent soreness experienced 24 to 48 hours later.  Soreness properly managed can translate into less injury to the muscle tissue in subsequent exercise sessions.  This type of proper muscle management leads to greater development over a shorter period of time.  

As a side note for those who are intensely interested in weight training for a sport, powerlifting or bodybuilding...static stretching BEFORE your workout creates a weaker muscle that is less capable of pushing heavy loads.  AGAIN, dynamic stretching is the appropriate form of stretching prior to a workout.