The whole point of warming the engine up (even for just a 1-2 minutes) in this way at the lowest possible speed of idle speed is to avoid the situation as much as possible where different components of the engine increase in temperature at different rates which increases the wear and tear resulting from friction between them. The source of heat in the engine is the combustion chambers where the fuel/air mixture explodes.
Components of the engine increasing in temperature at different rates also increases the possibility of the mating surfaces on components warping and causing mechanical problems, such as a leaking cylinder head gasket. By allowing the engine to warm up at the slowest rate possible, which is at idle speed, the risk of this happening is minimised.
In general, wear in the engine is minimised by allowing the engine to warm up at the slowest rate possible in this way. Conversely, the harder one drives a cold engine, the greater the temperature differences between engine components becomes. This increases the rate of wear and increases the chance of damaging the engine.
