When a petrol or diesel engine overheats, usually from a lack of coolant caused by things such as water pump failure or a radiator leak, the cylinder head often becomes distorted or wrapped. If this cylinder head is refitted without being refaced or skimmed, then the repair will often fail and the head gasket will not be properly compressed and a sufficient seal will not be created.
The operation of skimming the head creates a new flat surface and removes the risk of future failure.
In a modern engine, the tolerance of the cylinder head should be less than around 75 microns. This fine tolerance is because the head gaskets used in modern engines are usually multi-layered steel gaskets, which are much thinner than older-style head gaskets and require much greater accuracy.