As an example, your shoulders should allow you to reach behind your back and touch both hands together. If this is not possible, you are not as flexible as you should be. This does not give you a license to force yourself to do this stretch. Flexibility must be attained in a controlled and safe manner. Flexibility is likely the most neglected aspect of physical fitness and should be a part of everybody's weekly fitness routine.
Flexibility increases physical efficiency and performance.
A flexible joint has the ability to move farther in its range and requires less energy to do so. Flexibility decreases risk of injury. Flexibility increases blood supply and nutrients to joint structures. Flexibility training increases tissue temperature which in turn increases circulation and nutrient transport. This allows greater elasticity of surrounding tissues.