The holocaust is not necessarily unique as an instance of ethnic cleansing or genocide. But it was unique in that it was the first instance of industrialized genocide, and was carried out with a degree of bureaucratic process and precision not often observed. Thus it generated a volume of documentation that gives unique insight into how humans might be capable of these sort of atrocities against their neighbors.

Something similar exists for slavery (and Jim Crow) in the US, but we as a nation do not have the fortitude to confront our past in a way the Germans have.