Applications
A torque multiplier is a device that increases the torque that can be applied by an operator. Because the power output can not exceed the power input, the number of output revolutions will be lower than the number of input revolutions
(Torque x rpm = Power)
Safety
use of long levers can be dangerous.Torque multipliers mean a reduction in the lever length or operator effort by a factor of 5, 25 or 125.
Space limitation
the use of a long lever may be impossible due to the available space.
Accuracy
torque will be applied most accurately when it is applied smoothly and slowly.
Torque multipliers enable this by removing much of the physical effort from the
tightening task.
 

The adjustable reaction square can be moved and firmed on an adjacent screw with an impact nut socket and also to be a sustain point to torque multiplier.
The adjustable reaction square is optional once there's a sustain point to be reacted e.g. wall.