The body, brain and muscles are being deprived of carbohydrate fuel, and nothing functions well in the short term. For some athletes, carbohydrate restriction means substitution with a high fat and protein intake. However, even if you start that way, after a day or so most athletes will experience loss of appetite as their metabolism shifts to ketone production. Since it is both practically and physiologically difficult just to eat fat and protein, most athletes probably end up adding some carbohydrate by the end of the depletion phase, therefore not depleting properly. Athletes that do follow the protocol hope that by pulling themselves down hard, the next phase of loading will spring them back to new heights.
Some athletes are old-fashioned and stick to what previous legendary athletes and coaches did.
Perhaps they just read old magazines and studies. Many athletes are guided by superstition as much as experience. If an early competition success, or the success of a rival involved a depletion preparation, the athlete may want to follow what seemed to work. In a sport like marathon running, athletes may only compete at their highest level a couple of times each year. Therefore it is difficult to experiment with and assess the contribution of many different variables that affect the final result.