What was the overall purpose of the ghettos during the Holocaust?

The ghetto was like an animal cage, it contained what was inside it and limited contact with the outside world. This was ordered as part of the final solution to Hitler Jewish question. They were contained and catalogued so they could be easily tracked, produce goods for the German military and could be easily deported. However that couldn't have been the only reason for the ghettos, they spreading suffering, disease and despair. Could have it been used to weaken and kill the Jewish population? Nazis would have just considered it to be the most efficient solution but it is quite clear that the main purpose was to make the Jewish race suffer before they were killed.