There were many reasons given. Among them was that moving the Jews all into ghettos allowed non-Jewish persons to assume the businesses and goods/products that were formerly held by the Jews. Secondly, it put all of the Jews in one place so it was easier to dispose of them later.
The purpose of the ghettos were to get all the jews in one place so they wouldnt be able to run and would be easier to collect and "deport" to consentration camps. Jews at the time thought that mass murder wasnt even possible so they made it seem positive saying things like now we no long have to be around those who hate us or they just want to take our possesions.