In hybrid system, analog system with digital control, a voltage controlled things is very important since it can be interfaced by DAC from digital system. For radio tuning with digital control, a voltage controlled LC circuit is needed. If we can turn a knob of variable capacitor mechanically, microcontroller system prefer outputting a voltage rather than controlling a variable capacitor mechanically with a motor.
In reverse bias, a varactor diode uses a p-n junction and has a structure such that the capacitance of the diode varies with the reverse voltage. Electronic tuning circuit as in television or FM radio tuners is a popular application of the varactor. The capacitance of the varactor is varied by the DC control voltage, retuning the resonant circuit.