In every sport, their is a coach who increases the performance and knowledge of the athletes they have under them. IMHO bodybuilding is the same. As Mountaindog1 said, you can make all the mistakes that most coaches/trainers have made and in time learn for yourself..... though I have seen a bunch that would never get a grasp of everything they need to do to be successful. There is no sport, particularly at the pro level, but even at the amateur level, that does not have a quality coach to push and point out mistakes their players/students make. So if you are serious about any sport, you better get you a quality, well rounded coach, and in most sports it will be done for you. You can not play football, baseball, basketball, hell I can not think of a sport that you do not need a coach or trainer. I fought Full Contact/NHB/what is know as MMA today, and I had a coach/trainer, and I would never have achieved what I did without them.
I think some people get hold of a "coach" that does not have a clue what they are doing, and come to the conclusion they do not need one, and I can understand that. But it is not that you do not need one, you just need a qualified one. Mountiandog1 would be a great one for people to check out, as I know there are others that are very good too.
badham