Imagine a rubber band that has been sitting for a long time without being stretched. It becomes hard and inflexible and can easily snap if stretched too quickly. Your muscles work very similar to that. There are endless benefits, but the main reasons I would design a stretching program for a client is to 1. prevent injury and 2. help them correct muscle imbalances that affect the pressure on joints during an exercise. In conjunction with weight training, stretching can help the body's frame get back into alignment.
Here are some basic tips on making the most of a stretching routine:
1. Warm up 5-10 minutes before beginning.
2. Hold every stretch 30 seconds (If not using a watch, I count for 45 seconds to make sure I am not going too fast.)
3. Stretch each muscle group 2-3 times.