Each rep range is a good place to train, but each range has its own benefit…

high (15-20): builds muscular endurance

medium (8-14): increases muscular hypertrophy

low (1-7): builds max strength and power

The good thing about a high rep range is that you have an easier time mastering the exercises with a lower weight. All my workout cycles will start with this "easier" week, then proceed though the other ranges until you will finish on week six with low reps. This will most likely be your most challenging week, the reason it is saved for last.
No matter what your prescribed rep range is, you want to make sure that the weight is challenging enough. This means that you can perform all reps with good form, but the last 2-3 reps are HARD to complete. If you are aiming for 7-10 reps and don’t feel anything by rep 9, the weight is too light or your modification of the exercise is too easy. At that point, you will want to progress the movement by either adding a modification (for non-weighted exercises), or by adding more weight (for weighted exercises). Timed exercises will work in the same way. On weeks with high reps you should be able to perform the exercise for a long time, and vice versa.