The output impedance of a voltage regulator or DC/DC converter contains information about the control-loop stability as well as information about the decoupling network. 

The theory behind the Non-Invasive Stability Measurement (NISM) is explaind and verified based on a lab-experiment on a SEPIC converter.

If you want to know more about why measuring the ouptut impedance of a DC/DC converter can support your design process, please check out the following application note