An effective fitness program is more than aerobic training and strength building. To really reap the benefits of exercise, you need to add flexibility training to the mix.
Stretching can help your body get ready for exercise. It is also an essential part of recovering from aerobic activity. All exercise sessions should end with stretching - and not just for the mental relaxation benefits. The more flexible you are, the less likely you are to be injured during exercise.
A good warm up prepares your body for more intense activity.
Just as a warm up prepares your body for exercise, an effective cool down gives your body time to recover.
By increasing your flexibility you can improve your ability to move around. You will have less muscle tension and your posture will likely improve. Most importantly, stretching after each workout reduces your risk for injury.