Well Ioannis, Here is an excerpt form the data sheet:

The Numerically Controlled Oscillator (NCO1) module
is a timer that uses the overflow from the addition of an
increment value to divide the input frequency. The
advantage of the addition method over simple
counter-driven timer is that the output frequency
resolution does not vary with the divider value. The
NCO1 is most useful for applications that require
frequency accuracy and fine resolution at a fixed duty
cycle.

I have recently built and coded a closed loop PID control for a stepper motor with the NCO.