Fan control is necessary in HVAC systems for several reasons. Primarily, it allows for the efficient operation of the system, ensuring that air is circulated and ventilated effectively according to the specific needs of the space. By controlling the fan speed, it's possible to optimize energy usage, reducing electricity consumption and costs, as the fan can run at lower speeds when less air circulation is needed. This not only saves energy but also extends the lifespan of the system by reducing wear and tear on the fan motor and other components.