Power over Ethernet lets you run both power and networking to certain devices through one Ethernet cable. It's extremely convenient, especially if you have a managed PoE switch, because you get the following benefits:
- A single cable for power + Ethernet (no need for separate power adapters)
- No need to have electrical service near every device
- Simple remote power on/off capability (assuming you have a managed switch)
- Centralized power management (e.g. one UPS in a rack room covering all powered devices)
I have used the Raspberry Pi PoE and PoE+ HATs for years now, allowing me to have 4 or 5 Raspberry Pi per 1U of rack space, with all wiring on the front side. I also use PoE for cameras around my house, though there are dozens of use cases where PoE makes sense.
The Raspberry Pi, since it only requires 3-10W of power, is an ideal candidate for PoE, assuming you can find a HAT for it.