To identify them as Jews. Jews were stripped of their rights as German (and other) citizens by the nazis. Thus, when a nazi saw a Jew, he then knew to treat the Jew like an animal, not a person. Jews had curfews and could not do a lot of other things that non-Jews could do, like eat in certain restaurants or go to the theatre. It only got worse. As the war progressed, Jews were rounded up and thrown all together in ghettos.. . You provoke an interesting point, though. Allegedly, Aryans were the superior "race" (Judaism is not a race). Well, shouldn't the nazis have been able to identify Jews by their "race" rather than an armband?! Truth is that many nazis had a lot of Jewish blood in their families, which is what is so absurdly hypocritical about their philosophy.