Die hard over clockers want to push their CPUs to the limit. They will do anything to lower even a degree of temperature so they can over clock more. When you increase your CPU's speed it creates more heat and thus the need to install expensive cooling. The surface of the IHS** is not very smooth and sometimes is not perfectly flat to allow full contact with a heat sink. This causes slightly higher temperatures.. So over clockers sand their cpus, heat sinks and water blocks to a mirror shine by hand so that they are perfectly flat.
** IHS is short for Integrated Heat Spreader. It is simply a 3mm (AMD) piece of galvanize copper glued on to the CPU to spread the heat from the tiny core underneath it and to prevent it from getting crushed by coolers by spreading the weight.