Norton's and Thevenin's Theorems are used to simplify complicated linear circuits. They're applicable in any case one would like to simplify a messy linear circuit that is not already in Thevenin or Norton form.
Maximum Power Transfer:
"Practical applications of this might include radio transmitter final amplifier stage design (seeking to maximize power delivered to the antenna or transmission line), a grid tied inverter loading a solar array, or electric vehicle design (seeking to maximize power delivered to drive motor)."
