In my understanding, a CPU power supply is usually implemented by of a Point-of-load (POL) converter which is usually a buck converter. And this POL converter is required to have a fast transient response. It implies that the POL converter can adjust the output voltage to the required level whenever the current loading changes. However, I don't understand under which situations would a current loading of a CPU changes. Could anyone explain and give examples? Thanks.
The current draw of a CPU literally changes every single clock cycle: depending on what operations a CPU performs at any given moment, different transistors and more importantly, different amount of transistors switch.
Switching transistors is the main place where power goes.
Therefore, power supplies for CPUs have to deal with CPUs drawing different amounts of current, based on the different things a CPU might be doing.