In general, high rep (15 or so), low weights builds strength and muscular endurance with a minimal size increase. Pure strength increases are limited. Runners who are looking at weight training as a way to help prevent injury or don't want to add mass they have to carry during races often use this as their main weight training.
Low rep (3-8) high weight build power and size. Good for bulking up, but minimal muscular endurance. Powerlifters use this as their main area.
Middle ground (8-12 reps) builds strength, size and definition. People who are lifting to "look better" and gain bone strength advantages use this range as their main area. Most weight training books are written for these people, because they're the bulk of weight trainers in most gyms.This is how
fitness workouts works.