Typically used only on vehicles with diesel engines, the vacuum pump is required to supply a vacuum to the brake servo, which in turn provides extra braking power to the brake pedal operation. It is also used to power auxiliary systems in modern cars such as cruise control, or the turbo wastegate used in some turbo-powered engines.
Petrol engines do not require vacuum pumps, as the necessary vacuum pressure is created naturally by the operation of the engine. Vacuum pumps for every make and model of car are readily available and should be immediately replaced if found to be faulty.