The main purpose for pre-heating parts used for extracting aluminium is:
Remove water (liquid) from the goods so that the molten aluminium or cast iron used in the process does not heat the water to gas phase causing an explosion. Because explosions like this have happened in the industry in the past it is considered a must to pre-heat everything in order to not cause accidents.
Pre-heating the goods to working temperature allow for controlled freezing of cast iron or avoid freezing of the molten aluminium in the moulds.
Pre heating can also be used to melt out solid aluminium in order to clean parts.