It's one of the many factors contributing to health, absolutely. Is it the key? I think it can be because it usually leads to analyzing other aspects of your life as they relate to exercise. Obviously the method differs but typically people that partake in exercise are conscious of their health and may try to make better choices in other areas such as not smoking, drinking more moderately, eating better (and this is where it all goes wrong for most, thanks to misinformation) getting more sleep, more sun, playing more, etc. Exercise can be the beginning for many.