The Nazis were obsessed with not only the purity of the Aryan race but also culture. The documentary Architecture of Doom goes into a lot of detail about the Nazis and their obsession with art. The whole thing is available on YouTube and is a really interesting examination of Hitler's obsession with art. He was really seeking to remake Europe both racially and culturally and saw himself as the ultimate judge of what the end result should look like.

The growing Nazi collection of art was to reflect their ideals of what "good" art was and uphold Nazi ideals of racial and natural beauty. After the war, the collection was to be a demonstration Germany's cultural superiority on top of its military and economic dominance.

I highly recommend the documentary as it is really fascinating and goes a lot further to answer your question but also ties Hitler's obsession with art into the Final Solution and Nazi racial policies throughout their time in power.