Electronic Circuits are used strictly as crafting components. They can be upgraded to Advanced Circuits to craft higher-tier items.
As well, a new and surely important part of advanced machinery are these new Electric Circuits, officially labelled EEEAAAUUU Multi-Functional-All-Around-Ultimate-Usage-Circuit.
If you want to craft your own machines, don't forget to build some circuits first!
This little thingys are highly advanced technology and can be used for any kind of electric control system! Anything past a basic Furnace can't work without them.