What is powerline networking and when is it useful?

It carries LAN data over a building's electrical wiring via powerline adapters — useful where running cable or reaching with Wi-Fi isn't viable.

Powerline networking allows devices to connect to a LAN through electrical outlets by sending data on certain frequencies using powerline adapters.

Useful when:

• Data network cables are not viable
• Wireless communications cannot reach all devices
• Wireless access points cannot cover the entire home

Go deeper:

• Power-line communication (Wikipedia) — describes powerline adapters carrying Ethernet over existing home electrical wiring (HomePlug/IEEE 1901) and when it beats running cable.