The brake booster’s main function is to assist the driver by decreasing the braking effort. The hydraulic brake-fluid pressure in the braking system needs to be at an adequate level to achieve the required actuating forces between the friction surfaces of the brake pads and discs (or shoes and the drums in the case of drum brakes). The most common system is the vacuum servo (pictured) that uses vacuum (air pressure) to add to the force applied by the driver on the brake pedal.
In a modern ABS system, the ABS pump can take over the pressurising task during emergency braking (brake assist) to activate ABS, or to prevent a collision when the driver fails to react (collision-avoidance system). Some new systems employ an electric actuator that replaces the vacuum booster. It’s a more compact system and allows for better control of the hydraulic-fluid pressure.