rotating phasor how to
Are you talking about the basic phasor used in introductory AC circuit theory?
Polar coordinates are sometimes more easily visualized and if the phase of the signal is important (like in phase shifters and MPSK modulation).
If the vector is rotating, it sometimes means that its frequency is slightly different from the reference frequency. It is gaining or losing phase.
In electric motors it is sometimes used to describe the magnetic field direction.