Without actually knowing, either this (PowerLine), or they use the existing resources (cable pipes, wiring posts, ...) to run fiber or other wires to their customers without having to invest in digging or putting up poles.

This saves not only the construction costs, but also allow speedy installation as the time needed to get digging/building permits falls away. It's dead fast to wire an entire city area with FTTH if you already have a network of pipes for electrical wiring.