The Nazi Party adopted official anti-Semitic policies because party members needed a scapegoat.
The Nazis blamed almost every single bad event in the recent history of Europe on the Jews and especially Hitler blamed them for the things that happened to German after world war 1. Since many Jews owned businesses while other germans were out of a job and didn't have bread to eat they quickly started to believe the things that Hitler was saying. Not everyone, of course, believed in that nonsense but enough did.
