In yoga, stretching is considered very important because it is supposed to improve blood circulation. This is especially important for the spine, because very rarely do we actually stretch the spine.
When we stretch the spine, the space between the vertebrae is increased. This is especially useful for those who have back problems, and even mild instances of spondylitis have been known to be treated with simple stretching.
When we stretch, especially when the stretch is to the maximum, we pull and push internal organs, stimulating them. In fact, stretching on waking up not only relieves any kinks in the body, but also increases blood supply to the whole body, helping us to get alert.
Another advantage of stretching is that it keeps cartilage tissues flexible. Cartilage and tendons are very strong tissues, but a certain amount of flexibility is inbuilt in them, which is what allows us to move easily. As we age, the blood supply to these tissues reduces, hardening them and reducing their flexibility. Stretching ensures that you renew blood supply to the cartilage and tendon tissues, keeping them supple and strong – the reason why many top athletes stretch regularly is to keep their hamstrings and Achilles tendons in good condition. No studies have been conducted on whether cardiovascular efficiency increases on stretching, but studies comparing regular stretching and flexibility exercises with yoga have showed that they are as good as yoga in many instances. Yoga is considered better because of other benefits, but purely from the standpoint of improving overall bodily health, stretching is more than sufficient.
A good stretch invigorates and refreshes us, improves blood supply to all our organs, and relieves a number of physical ailments, not least being flatulence and constipation. If you don't have to spend too much time on stretching exercises, even 10–15 minutes a day is often more than sufficient for most people.