Batteries for example, or you may have power from other circuits that you can tap. A light circuit with multiple switches requires comparatively complex wiring

Batteries for example, or you may have power from other circuits that you can tap. A light circuit with multiple switches requires comparatively complex wiring - either all switches need to connect to a central point (breaker box) or you need to have some sort of loop that passes by all switches. It’s nontrivial to retrofit. Some sort of (wireless) bus system that transmits the signal digitally can help reducing that complexity.