I just talked with a good friend and professional baseball player about the times when a good coach is needed the most. The slope of improvement is always greatest when a coach can relay the most relevant cues in the most efficient manner.
However, this slope of improvement is most important in beginning and advanced athletes. As a beginner, a thorough introduction to new movements and its most vital aspects are necessary. After this intial period, the athlete must develop a solid work capacity through multiple sets/repetitions to “groove” their technique. Once a solid work capacity and technique are developed, the focus must shift to more complicated exercises (i.e. ones that emphasize more speed and more weight). During this advanced training, an experienced coach and his/her program is essential to allow continued gains and avoid the risk of injury.
You can only do lunges and step ups for so long.