The grinding of the plane of the motor head or the flattening of the head is a very important step, but not all heads can be flattened, or at least you can grind the plane for a few hundredths of a millimeter.
Some heads are deformed due to overheating and lack of cooling, this problem causes the top to bend, which in this way causes the engine gasket to burn.
The head flattener is a machine tool that smoothes the surface by removing it and removing the bare minimum to bring the surface of the head perfectly flat and smoothed with the right polishing or roughness.
Today many dismantle their heads without checking the plan, but only by cleaning it. It is absolutely necessary to check the flatness with a special rigello and if necessary to flatten the surface even very little the surface, because today’s heads are tight at such high forces, that just disassembling them cause deformations that do not guarantee surface sealing between the block plane engine head and cylinder head.