For monitoring of aggregate loads (e.g. at service entrance, substation, etc.), the parametric plots allow for an easier determination of load impedance.
Impedance can be estimated on a regular waveform capture graph by picking a point on the voltage and current sine waves (at the same time), and dividing, but a more accurate estimate is possible by using the parametric graph, and using the slope of a linear portion.
The vector diagram can also be used for impedance calculations, by dividing the voltage and current phasors, but this assumes a constant impedance throughout the waveform, and ignores harmonics.
Parametric graphs are a less common method of viewing waveforms, but can reveal details not obvious standard time-series displays.
They are especially useful when monitoring specific complex loads, and can be compared to reference signature plots to reveal changes in the equipment, or problems.