Thévenin equivalent
Any linear circuit, seen from two terminals, behaves exactly like one voltage source in series with one resistance: Voc and Rth. The whole network collapses to two numbers that predict what any load will experience.
In practice
It answers the practical question 'what happens when I connect something here?' — sag under load, maximum transferable power (load = Rth), interface stiffness. The editor computes it live for any net: click and read Voc and Rth.