The purpose of stretching is to warm up the muscles and get the circulation flowing before exercise. This may help to prevent injury and it is a good way to improve posture. Many animals also use stretching before they start to run or walk, particularly seen by cats and dogs.
Yes, it is necessary to stretch before exercising. Stretching helps to lengthen the muscle fibers and to relax them. This helps to prevent them from tearing due to the contractions caused by exercising. When people don't stretch, they are likelier to experience soreness. People who do not stretch are also at a greater risk of injury while exercising.
Stretching before exercise helps to ready the body for the physical exertion that is going to be required to attain the desired level of fitness. Without stretching you risk injuring yourself and causing damage to the muscles and ligaments that are being worked. Stretching helps to ready the muscles making exercise easier and more effective.
No, exercising warms up the muscles, and the old practice of stretching beforehand has proven ineffective for most athletes. Stretching can actually negatively affect performance and benefits those who require flexibility in their activties. Runners and cyclists can warm up without stretching. However, stretching after exercising when the muscles are warm can be beneficial for the average person who performs Yoga, because it increases flexibility.