The term 'accessory' to describe standard bodybuilding movements began being used in the field of strength sports. The implication of the term is that developing a main competitive lift, such as the squat, is the main focus and that all other work is done to build up weak points that may hinder progress in the main movement or lead to injury. The concept of accessory doesn't apply to, say, a bodybuilder because the entire work week is already spent picking apart muscles in the body and training them independantly of others. To a bodybuilder, compound movements still provide the backbone of a solid mass routine, but they do not get any higher priority than the isolation work that comprises much of the workout.
Accessory work, or as the rest of the fitness world calls it, 'working out', is essential for building a large and physically capable body. The point of the whole thing is to isolate individual areas, be they muscle groups or entire energy systems, keeping up odds and ends that will be left behind otherwise. It stands to reason that a max effort lift will fail based on the weakest link, and if every link is given equal attention, then the chances of a weakness developing is minimal. Special consideration should be taken when planning lifting blocks to prioritize which areas are holding back progress the most. That being said....
Here are the 5 reasons for choosing accessory work:
To Work and Antagonist
Muscular contraction doesn't simply serve to move a limb through space: much of the work that muscles do in the human body is spent stabilizing and providing tension AGAINST the working group. Take the knee for example. In a squat, the quads work to extend the knee while the hamstrings work to extend the hip. But the two-joint hamstring also anchors below the knee joint, creating 'flexion' force in the opposite direction of the quadriceps extension force. This may seem like a hindrance to optimal squat performance, but the extra stability derived from the opposing tension keeps the knee from deviating out of position and makes activities like running, jumping, and squatting much more efficient.
Antagonists, while vital to the efficiency of the lift being trained, do not develop the same way the main movers do; for instance, bench pressing will not build your biceps. When all of the main work for the day is said and done and the primary movers are smoked, prioritizing antagonists will encourage healthy movement mechanics and keep the working joints stable over time. As strength progresses and the joints are subject to heavier and heavier loads, balance between opposing muscle groups becomes paramount to success.
To Work a Main Mover
Competition lifts will hinge on a few main movers that are responsible for driving the bar through the complete motion. A world record bench press cannot allow for any weakness in the chest shoulders or triceps. You can't deadlift big with small glutes or squat large with skinny legs. A common mistake that happens as lifters break out of the novice division and graduate to the ranks of intermediate is the exclusion of most movement variations in favor of just the big lifts. In the beginning, the refined movment patterns and increased efficiency that comes from practicing the main lifts over and over and over can greatly out weight the disadvantages from not including accessory work. But this only lasts for a time. After a few years of hammering away at the basic compound movements through the same basic rep ranges, stagnation hits.
All lifts can benefit from extra volume on their respective movers in the form of varied accessory work. Triceps are a prime example; benching requires the triceps to be strong, but the lift itself doesn't expose the triceps to loads it is ultimately capable of handling at the top end where triceps' action is dominant. Because the weight used in any given exercise is determined by mechanically the weakest range of motion (off the chest, in this case), the top end is never loaded appropriately and the triceps never receive a stimulus for substantial growth. For this reason, the best benchers in the world, with few exceptions, pin their success on the inclusion of a few personal favorites for hammering the triceps.
To Prevent Future Weaknesses
We discussed how we can incorporate accessory work to fix relative weaknesses, but what about never dealing with weaknesses to begin with? An interesting Facebook thread popped up some time ago with a well known Crossfit coach. The topic at hand was 'which athletes would transition to Crossfit the easiest?'. This is relevant to our discussion because top level Crossfit athletes must be well rounded and void of any single individual weakness. Dozens of replies came in, advocating for football and rugby players, track athletes, and Olympic weightlifters. The OP's recommendation? Bodybuilders.
The reasoning was that bodybuilders handled high training volumes, kept rest periods short, and would transition from barbell movements to bodyweight easily due to a well rounded physique. This would make them prime candidates for a sport that requires excellence in a high volume of activity using varied movements and training thresholds. The moral? Staying well rounded will prepare you for anything.
In a much more focused approach, we can take the 'stay ready for anything' mantra and whittle it down to a small harem of exercises that can prevent the most common struggles that develop for blooming lifters. The standard Westside template, along with countless other powerlifting protocols, follows up a normal bench day with exercises for all muscles used during the bench press: chest, shoulders, triceps, as well as the biceps, lats, and rear delts. By including secondary movements to build up smaller antagonist groups like the rear delts before they become a weakness, progress continues for longer before underdevelopment leads to stagnation. This 'no man left behind' approach is one of the main reasons bodybuilders can develop world class strength without specifically training in the strength threshold.
To Prevent Dysfunctional Movement Patterns
Some accessory exercises carry an added quality of increasing flexibility and healthy movement patterning and might be programmed specifically for those reasons. Look at any competitive lifter who has spent much of their time running through the big 3: as they grow in size and strength, the tightness that encourages big squats and presses can also hinder normal healthy movement patterns. I have met massive benchers that don't squat because they physically cannot get their hands on the bar and 1000lb squatters exist who can't deadlift their way out of a paper bag due to poor setup caused by limited movement capabilities.
By frequently rotating in exercises that take your body outside of the same three directional patterns, function can be improved, both in and out of the context of the competitive lifts. Romanian deadlifts are excellent for putting mass on the posterior, but also encourage plenty of flexibility through the normally bound hamstrings. Good mornings reinforce posture under a load, dumbbell presses allow the shoulder joint to work through greater ranges of motion, and unilateral leg work builds stability throughout the knee and hip. You don't have to be a jack of all trades, but it would serve your long term game well to maintain basic mobility through varied exercises.
To Train Another Skill or Energy System
So far, the discussion of accessory has centered around the development in size and strength of muscles that contribute to a main lift. However, sometimes it is a separate skill or energy system all together that needs priority. A muscle can still lag on rate of force development even though it is well developed, and the training of a dedicated lifter can still suffer due to poor aerobic conditioning.
Take a squatter who is a proverbial tortoise; always getting the bar from point a to point b but never in much of a hurry to do so. A variety of weighted plyometric exercises can be ideal for conditioning more motor units to fire at once, improving starting speed and increasing the likelihood of completing the lift. For lazier lifters, occasional work in the cardiovascular range can lead to reduced recovery time between sets, boosting overall training density. A popular Westside recommendation comes in the form of GPP (general physical preparedness), essentially stating that being out of shape is a hindrance to elite training. Off days are typically filled with light feeder sets, bodyweight exercises, hill runs, and oh yes, sled drags. Sled drags became a staple in the world of powerlifting for their vicious 1-2 punch of building the quads as well as general anaerobic conditioning; both essential for marathon squat workouts.
Even athletic improvements by way of agility and coordination drills can improve basic kinesthetics that will lead to more self awareness during technical work. Incorporate a basic amount of speed, agility, and conditioning work and watch the efficacy of your workouts skyrocket.