What better way to learn about the horribleness and the greatness in the world? What better way to learn that even though there was hate there were still good people left in that part of the world to help. Teaching about the Holocaust also helps to understand the hate pyramid. Not only can we see the different stages, but we might be able to connect it to our surrounding world. This also helps teach about the abuse of power. Which there may be lots of in this world.
There are also lots of other opinions on this