A microprocessor is a digital device that deals with only digital inputs & digital outputs (0, 1).
ADC & DAC are used to enable microprocessors handle analog or continuous signals.
The larger number of bits used, the better the resolution becomes.
It is necessary to have control over the voltage that go to each motor and to be able to read the feedback signals from each robot joint.
Imagine how many input & output ports would be necessary to have an accurate robot control.
PWM (Pulse Width Modulation)
PWM is used for DC motor speed control with microprocessors (microcontrollers).
PWM is a technique of creating a variable output (voltage) that requires only one output bit of information.
The voltage is turned on and off repeatedly so that by varying the length of time that the voltage is on or off, the average effective voltage will vary.