Mobility: how a joint moves.
Flexibility: length of a muscle.
Because Mobility requires your body to actively take the joint through it’s full range of motion, static stretching will not help improve mobility. A good example would be in someone experiencing restricted ankle mobility and trying to stretch the calves. Stretching will address a part of the problem (tissue elasticity) but not address the complete picture. Most limitations come from poor joint mobility and motor control, not the length of the muscle.
Controlled movement and motion through end-ranges of a joint is needed to fix a mobility issue.
When the goal is to improve range of motion for a movement or exercise, it must be treated like all other types of training. Connective tissue needs to be placed under mechanical stress to remodel the tissue being addressed. Long term change will not happen until the body adapts to that stress and feels safe going into the new-found ranges, not from it being forced there.
Active mobility drills are the best warm-up to a training session. They prepare the muscle and joints to receive load. It will create readiness and activation through muscle tissue to create more power and reduce the risk of injury. Conversely, static stretching prior to exercise has been shown to decrease performance. This makes sense when you think about it. Static stretching is taking the joint to end range with no regard on how to control it. It will also stimulate receptors in the tissue that are designed to create inhibition, or temporarily shut down and weaken that muscle. Not so good when you are preparing for an intense training session or sport.
We can obviously see the importance of incorporating more “Mobility” training, but you might be asking, when should I do static stretching. Simple, anytime you are looking to inhibit a muscle or decrease its tone. Good example would be stretching the hip flexors from being seated all day.