The last, almost impossible repetition of each set is the most productive in terms of making your muscles bigger and stronger. If you never perform that rep, you have not “flipped the switch” to stimulate results, no matter how many sets you do. You can’t make up for doing one set wrong by doing 4 sets wrong.
In order to maximize your fitness potential, you need to regularly attempt the momentarily impossible. You need to push to your current limit in each workout, forcing your body to adapt to this new stress by building new muscular strength and size—your “exercise suntan”.
Tip: Always strive to do more reps, more weight, or both, on every exercise, every workout.