It is to power external devices, mainly the Pro series using only 1 power supply.
However, the DC loop is not 15V/4A. It is the difference what the powered unit consumes and what the external power supply unit can supply.
For example, idle the Pro iCAN/Pro iCAN Signature consumes 22W, so only 38W of the 60W the iPower Plus/iPower Elite (15v/4A) can deliver are available to the looped device, less if driving difficult headphones with high power.
