You would actually run VAR/PF control **instead of** voltage control.
With a regulator in normal voltage control mode, it is adjusting the excitation to try to maintain a setpoint voltage on the generator output (or at whatever point you connect to the voltage input). If the generator is running isolated, off the grid, this makes sense.
However, consider the situation of a generator tied to the power grid or another larger generator. The single generator is not large enough to change the grid voltage in most cases. So even a very small voltage error signal will cause the regulator to drive the excitation to maximum (or minimum) trying to get the voltage input back to the setpoint, but the generator just can't do it. So the generator excitation will swing from min to max as the voltage varies. For these situations, you must switch from voltage control to Var or PF control. In these modes, the setpoint is the kVAR output of the machine or its output power factor. Now the voltage can vary (within limits) and the regulator will be happy and making excitation adjustments to maintain a constant generator power factor or VAR output.