Cargo is a device designed to allow you to change the load impedance seen by your ribbon and dynamic microphones. This way you can affect the microphone frequency response, or optimize the matching between mic and preamplifier to fully exploit the sonic characteristics of your microphone.
The physical characteristics of magnetic transducers make it so that their electrical impedance change with the frequency of the reproduced signal. Therefore, by changing the load impedance seen by the microphone, you can modify its signal and frequency response.
Also, through a switch placed on the rear of the unit, Cargo can boost the level of less sensitive microphones, as for example ribbon mics.
All of that via Phantom Power, try it out! …you will not regret it!