In what regards? I am going to assume you mean in weightlifting exercises.
Generally lower reps are done at higher intensity (more weight) which tend to be very strength oriented.
Higher reps traditionally are employed to build muscular endurance and hypertrophy. Crossfit employs high rep lower weight exercises in combination with gymnastic and conditioning components. Rest is minimized if not avoided during the wod. The high intensity of these workouts in combination with no rest causes a huge neuroendocrine reponse that improves conditioning, muscular endurance, but also muscular strength at rates otherwise improbable.