When training, we do multiple sets of the same exercise instead of just doing one set. We do this because when doing multiple sets, you are doing more work so it is more beneficial to you. Also, the more sets you do, the more tired your muscles get, so they have to use more effort to fit the bar each time. You only start to get stronger when your are tired. Therefore, we do multiple sets because if you do one, you are not getting enough work out of the exercise.