An obvious question is "why not just use Power-over-Ethernet?"

Well, technically this module is essentially a passive PoE device following the 802.3af mode B pinout, but without adhering strictly to the spec.

When looking at passive PoE devices, you'll typically find something like this:

The problem with these cables is they only work with devices which are already networked, such as IP cameras and phones. Moreover, if the devices don't support the voltage received from the injector, they could easily be damaged.

In the situation where you want to control some 5V LEDs from a long distance using a wired setup, you'll need a DC-DC converter and an RJ-45 connector at the LED end, and you'll also need an RJ-45 connector near the MCU. Finally, you'll need some kind of device to send your data signal as a differential signal, assuming you want it to arrive intact at the destination - that's where the RS-485 transceiver comes into play.

In the end, a DIY setup to support "PoE" with non-networked devices will end up looking exactly like  this module.

Note: I didn't discuss "Active PoE" because that involves a bit more hardware and a lot more complexity.