Electrical wiring of any sort are all antennas to varying degrees, transmitting and receiving electromagnetic signals. Most mains electrical wiring is also unshielded, meaning they readily transmit and receive electromagnetic signals.

Powerline ethernet basically puts ethernet data on mains electrical wiring by utilizing the bands that aren't used for carrying power. This data is very, very noisy in electrical noise terms, and because most wiring also acts as an antenna that noise also gets broadcast everywhere.

Simple electronics like your coffee machine or microwave oven won't care, but more sensitive electronics like radios can in turn receive interference from both the power line and the noise broadcast into the air.