Accessories
Typically, there are two types of powerlifters in regard to accessories; those that undervalue them and those that overvalue them. I have gone through periods of time where I have done both. In most cases, I think powerlifters undervalue accessories and perform these exercises lazily and without intent.
Accessories are a vital part of any intelligent powerlifting program. They are there to build muscle, reduce the rate of injury, and improve upon the imbalances powerlifters develop as a result of so much squatting, benching, and deadlifting. The powerlifts are great at building muscle and strength, but they train some muscles significantly more than others, which is one of the reasons accessories are implemented.
In my opinion, some of the muscle groups that must be specifically targeted, as a result of not receiving enough stimulus from the main lifts are: upper back, biceps, and the medial and posterior deltoids.
Some of the primary and secondary muscles used during the competition lifts include the chest, triceps, posterior deltoids, lower back, glutes, quadriceps and hamstrings. In my experience, these muscles should still be trained with accessory movements but extra care should be taken to ensure they are not overworked. These muscles are already being trained frequently and training them too hard with accessory exercises may impede recovery and affect performance on the competition lifts.