Why tan-delta capacitance test required?

In a pure capacitor current leads the voltage by 90 degrees. when a resistor is

introduced in between the current leads an angle less than 90 degrees.

Similarly in a Dielectric material when a cavity or deterioration starts , the life of the

material starts deteriorating , as there is a resistance getting added and hence

leakage current increases In tan Delta we find the difference in the angle and

periodically note down the pace at which deterioration takes place.