One of the main reasons why auto brake is used as opposed to manual braking is that the Pilot In Command is required to maintain directional control (nose wheel steering) using the rudder pedals upon roll out.
And manual braking requires the application of toe brakes, so it is not possible for the Pilot in command to apply rudder and the application of toe brakes simultaneously.
On light aircraft (where auto brake is not a feature), differential braking using the toe brakes allows the pilot in command to apply directional control and braking simultaneously in the landing roll and taxing.
A light application of brakes on each of the main gears (left/right wheels) allows the pilot in command to make directional control on the ground during the landing roll and taxing.
During take off, nose wheel steering is used to prevent burn out of the brakes and directional instability when higher speeds are achieved (this applies to both GA and commercial aircraft).
On passenger jets with a multiple landing gear and higher mass, the application of toe brakes on the either of the main gears is deterred during normal procedures for obvious reasons (wheel damage/fire/burnout) and is the least efficient method of steering during the landing roll.
The application of nose wheel steering is used at higher speeds (wheel spin beyond 40 knots or more). And the application of the tiller is used during taxing at lower speeds due to it's ability to make smaller directional changes to the response of the tiller.
However in non normal situations, the crew are able to maintain directional control of the aircraft when the use of nose wheel steering is prohibited (nose wheel fails to retract/loss of hydraulic pressure, emergencies).
The auto brake uses an air to ground switch which makes certain systems within the aircraft redundant depending on whether they are in the air or on the ground. The spoilers are redundant in the air, but when the main gears contact the runway or at a given wheel spin the auto brakes are automatically applied usually simultaneously with the arming of the speed brakes.
And as already pointed out, the unique feature of auto brakes as opposed to using manual braking (ignoring the obvious pointed out earlier) is that it allows a consistent application of braking in response to runway and weather conditions.
Usually there are a few numbered braking options plus an anti skid feature for slippery runways, again these are there to reflect the runway length, conditions and weather.