This unit is typically used in 2 scenarios:
1 The mixer in use cant supply phantom power and you want to use phantom powered mics / DI boxes. Simply connect the mics to the Phantom Power Supply (PSU) and the PSU to the mixer.
2 The mixers phantom power is not individually switchable per channel and you are connecting some items that need phantom power and others items that will malfunction if fed phantom power. In such instances leave the mixers phantom power switched off and connect the items that require phantom power to the mixer via this PSU.