Well you don't, it just helps, a lot. You want to compete at a professional level, say in sports, no requirement to have a coach. Most group sports have them, but as for single events, you are just fine going at it alone, if you know what to do and how to train to improve yourself then you likely be just fine. If just fine isn't good enough, then a coach might be helpful. Someone who has been through it before and is only concerned with helping others navigate and train.