To improve flexibility, stretching exercises are most productive when they are performed

Stretching exercises are most productive when done after the workout or exercise to improve flexibility. 

During workout or exercise, lactic acid builds up in a person's muscles, giving that feeling of soreness and fatigue. The muscles can stay in this "semi-contracted" state for hours. Stretching after exercise helps to reduce this soreness by increasing blood circulation, helping the muscles to recover faster. 

In addition, consistently working out and exercising may increase the growth of muscles but the connective tissue (fascia) does not grow as fast together with the muscle. This means that little by little, the muscles will eventually become compressed by the fascia as they continue to grow despite the slow rate of expansion of the fascia. Stretching can help expand the fascia to allow for more room for muscle growth, without negatively affecting a person's flexibility. 

That would be a working theory.