Stretching is very important just for general physical maintenance, not necessarily related to riding a bicycle. I have developed a few aches and pains (knee and neck), gone to the doctor, and come away sometimes with drugs and sometimes without, but always with a list of stretches to do. When you have things like repetitive stress (biking) and inactivity (sitting at a desk, or bent over on a bike), things get stiff, or unbalanced in interesting ways, or pulled out of proper alignment, and pain can develop.