From Unofficial Stationeers Wiki
Many devices provide data to the data network, or may be written to, but circuits are special because they can interact with other devices.
Circuits are a key component of the game and are used to automate systems.
Circuits operate on data signals passed along cable networks. Cables can transmit both electrical power and data at the same time. Devices connect to the network when a cable is attached to the data port. The cable networks work as a bus, with all objects on a network visible to each other. Objects with two data outputs, most notable of which is the Area Power Controller, generally form two discrete networks.
Circuits run twice per tick, i.e. every 500ms, which is important for timer circuits and advanced applications.