Over the years I have used several different rep ranges and training methods.  From Super heavy weight/Low rep sets of 1-6 and Heavy weight/medium rep range to failure at 12-15, where I have noticed my best gains.  I don’t consider 12-15 to be a high rep set per se. It allows you to use heavy weight in a controlled manner and getting a sufficient number of repetitions in to pump the target muscle full of blood.  What is the pump? Each repetition causes a stretch and contraction in the muscles, forcing blood and nutrients into them, causing them to elongate, swell and the muscle fibers to tear. If the trainer’s goal is to increase muscle mass, then the main goal of each weight training session should be to create the biggest pump possible in the target muscles by employing a number of effective isolation motions.  The use of these motions to create a voluminous pump, should simultaneously fully fatigue and tear as much micro muscle fiber possible.  In nature, each stimulus has a response. The response by the human body to resistance training, is to add an increased amount of muscle to repair the damaged area to compensate for the amount of stress placed on it during the previous training session. This is the reason we build muscle-so that the body does not have to work as hard at the same task each time.  If the amount of resistance is increased along with variations in the exercises from session to session, the body will continue to adapt and add muscle mass as a result of this.  However, if the stimulus remains constant, the body will become conditioned to it and over time the increase in muscle will plateau.  
It should also be noted that during the training session, the body is pumping 4-8 times its regular amount of cardiac output (blood volume) from 5L/min to 20-40L/min and is an optimum time to intake nutrients which are beneficial to enhance pump, growth and recovery.  Furthermore, intake of properly timed vitamins/minerals and nutrients before, during and after training will optimize results, in addition to adequate amount of recovery time between weight training sessions.