PE is one of the Principles Joe Weider claimed as his discovery, nonetheless PE is a great method for several reasons.
PE & Drop Sets are both great for the aging athlete seeking to avoid the risks associated with heavy weights. For instance, I can burn up my quads and hams prior to pressing or squatting and attack muscle fibers with less weight and produce great trauma with medium weights, hence giving my joints a break. All body parts/muscle groups can incorporate this practice of fatigue via isolation and then performing compound movements.
Drop Sets is another way to deep punish muscle fibers by starting with a set of 10 and dropping the weight with no rest and performing another set and another until the muscle is spent.