The Alternator's duty is to produce Electrical Power to run the engine and other Electrical Equipment and charge the Battery. The voltage range produce by an Alternator is 13.3V to 15.3V.
You may notice that the Alternator does not produce an ideal fixed Voltage, this fluctuation of voltage is called ripper. This ripper will cause the voltage signal of the sensors and actuators to fluctuate as well, be it that it had been transformed to 5V range for the Electronics.
The Voltage Stabilizer's duty is to minimize this fluctuation of voltage. The effect of this will be more crisp signal to and from sensors and actuators. This will allow the ECU to more accurate in term of decision making. Therefore more efficient engine output.
There is a better version of Voltage Stabilizer which combines with Earth Cabling and it is install between the Alternator and Battery. It acts as a charging regulator for the Battery as well and will prolong the live of this battery.