Quite simply, to dispose of the casualties of war in a sanitary, sensible
manner, especially in view of the many contagious diseases that were rampant
in some of the camps such as typhus.
Burial meant contamination of the ground water which endangered everybody,
not just the inmates of the concentration camps. Therefore, people were
cremated.
There is nothing sinister about crematories found in concentration camps.
They protected the living from dying - the concentration camp guards and
the administrators as well as the inmates.
The situation would have been far worse, in terms of casualties, had there
been no crematory ovens.
Nizkor's reply is both shoddy and snotty. Very little else needs to be
said.