Thevenin is thee most used theroem in all of circuit design. It reduces any circuit, no matter how complex, to a real generator, with it's characteristic internal impedance. It is used to determine how different loads will effect a signal source output. It is also used in transmission line drive calculations. It is also used to determine how long it will take a digital signal to go down a bus or a backplane. Learn it, and keep it in the front of your forehead.
If you drop a cresent wrench across a car battery and measure the voltage at the instant of contact, is it zero?
If so, why?