Why is the error in control system calculated against input?
Normally a control system uses a setpoint voltage to amplify power, to achieve a position, velocity or acceleration with feedback to correct the error. There are many situations where power is used to control something and it is best if that feedback uses a sensor to convert from the same units that the voltage represents.
This means having the ability to use linear control to compare without having to integrate and reduce bandwidth,  add latency or use phase compensation for stability that may cause overshoot or ringing from disturbances or step changes to the desired input.
Yet often this is not possible and multiple targets are needed like acceleration, velocity, position or direction and multiple sensors are needed.