Before we can discuss the value of accessory exercises we need to spend a minute discussing the difference between core exercises and accessory exercises. These are the big movements that involve multiple joints and muscles working in coordination with each other (squats, presses, pulls, etc.). Due to the large number of muscles involved, core exercises give you the most bang for your buck and are the key exercises that drive your performance. You can think of them like the engine in your car.
Accessory exercises typically involve fewer muscle groups or joints working together and focus on stabilizing muscles, muscles that support the core exercises, or help with injury prevention. While they may not be the main drivers of performance they play a vital role in supporting performance improvements. Consider them the regular maintenance you perform on your car-oil changes, spark plugs, filters, brakes. While your car will continue to run (at least for a little while) without regular maintenance, eventually it will break down. Your performance can be viewed through that same lens. Without regular maintenance, performance will decline or injury will occur.