Why were jewish people in areas of europe occupied by german ""resettled"" to eastern europe?

During World War II, Nazis forcefully moved Jewish citizens to Eastern European territories, where they locked them down in walled communities called ghettos.

These ghettos had bad living conditions, people were overcrowded and didn't have enough food.

The first idea was just to gather people in ghettos and separate them from the other "Aryan" population, but later in the war, ghettos served as gathering places from where the Jews would be transported to concentration camps. Nazis promised Jewish people they will be transported to places with better conditions and opportunities to work, but were sent to the camps.