Its just extra training. Its not always more important, but it gives you an edge and the experience to cater your practice towards a specific speciality. Some will only work within that subspecialty while others use it as an area to gain experience in something they enjoy. I know an orthopedic surgeon that did a fellowship in foot and ankle, but his practice was largely general ortho. Along the same lines, my dad did a fellowship in cardiovascular radiology but he largely did general rads stuff with a few more procedures than usual. The ideal residency program will teach you everything you need to know to do anything you encounter, but this is the real world. In order to strengthen the parts you are weak in, get privileges/academic positions, have the edge over your competition, and to just be a better doctor you can choose to pursue a fellowship. Plenty of people don't pursue them and do fine. I'm sure there are orthopedic surgeons somewhere in the country that do spine stuff without the fellowship. I know many neurosurgery people don't pursue the fellowship. They feel comfortable enough without it oftentimes.
