Rotary potentiometers and varistors are important components for adjusting resistance and current in circuits. Rotary potentiometers are used for voltage division and voltage regulation by changing the resistance value through a rotary axis, and are commonly used in electronic devices such as volume control and brightness adjustment. Varistors, on the other hand, are used to regulate current, controlling the flow of current by changing the resistance value, and are used in scenarios such as motor speed control and heater regulation. These two devices work together to optimize the operation of electronic devices and improve system performance and reliability.