Correct me if I am wrong, but I think because they are also referred to as "crush washers", that when the injector is reinstalled and torqued, they deform to provide a seal between the injector and precombustion chamber. If the injectors are removed, there is no guarantee that the faces will align the exact same way before the injectors were removed and thus provide a proper seal. As well as they are a minimal investment compared to new or rebuilt injectors. Thus the mantra "replace, replace, replace...".

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1987 300SDL
2005 Ford Excursion 6.0L Powerstroke, 6" suspension lift