When a muscle is isolated more concentration, effort and focus can be directed when sets and repetitions are performed giving you better results. Cheating reps have their own place and benefit in training programs. You may not recognize it, but it happens. This can be frustrating, and even causes people to quit exercising. A foolish move.
Variety is the spice of life.
Remember that every 6 to 8 weeks or less, some changes need to be made in your program so that your muscles will continue to make progress and be working towards your goals. Performing the same exercises with the same sets and repetitions, while using the same equipment, makes the muscles stale and lazy. Switching around gives the muscles a jolt and keeps them developing. This type of training will cause an isometric response that will give an increase in muscle separation. This is another way to continue to work your muscle fiber after you have finished performing full repetitions, or if you have physical limitations such as limited mobility and joint problems, doing partial reps can be beneficial.