muscles are mainly composed of contractile machinery. training in the 8-12 rep range will cause mostly hypertrophy of this contractile machinery, or sarcoplasmic hypertrophy. therefore, there is most room for size gain here. training in a lower rep range will help increase the efficiency of your central nervous system, in addition to causing some sarcomeric hypertrophy.
training above the 8-12 rep range will lean you towards sarcoplasmic hypertrophy, or an increase in the efficiency of ATP production by your body. (some refer to this as "endurance training")
that being said, different people react differently to different things. experiment.