Why do some foods have to be cooked only when the pan is really hot?
http://en.wikipedia.org/wiki/Wok_hei has very good explanations about it. Let me sum of it up:

The geometry of the wok is very important:


better use of the surface area
ability to shallow fry big items and deep fry small items with small amounts of oil.
intense heat for low amount of fuel


The temperature is also very important:


caramelization (or http://en.wikipedia.org/wiki/Maillard_reaction)
carbon steel, cast iron, stainless steel and others have different properties in terms of heat capacity and heat conduction.