There is a big risk of injury if a muscle suffers a sudden and brisk stimulus. The athlete that starts an intense workout, without a proper warm up, is taking an unnecessary risk of injury.

It is important to remember that leaving out a warm up will not cause injury in 100% of the cases, but it will greatly increase the chances of it happening, and, sometimes, even well warmed up athletes suffer an injury. 

So, the warm up is a way to prepare the body for the effort it will suffer and, during the warm up, the athlete may, or not, do a stretching session. The stretching itself will not, in a direct way, alter the performance.

Light exercises, like slow pace walking, don't require a previous warm up. In this case the warm up happens during the walking. There is no need for a warm up before a low intensity aerobic activity.

The stretching after the workout is used by most athletes, since it is believed that the stretching acts like a relaxing stimulus for the muscles that were working hard for an extended time. It is important that the athlete knows that stretching is not advisable if the exercised muscle is injured.