Because it changed the world as we know it whether politically (European Union and NATO), militarily (the SS was the model for a Pan-European military and squad tactics), world justice (Nuremberg trials and how nations approach crimes against humanity) and science (first man-made object in space, first operational jets and a myriad of other items of interest)
That is of course just a tip of the political and historical iceberg. WWII is also interesting in an anthropological way.