After World War II, the Allied Powers took joint control over Germany. This includes Berlin. Eastern Berlin was controlled by the Soviet Union while Western Berlin was controlled by the United States (along with Great Britain and France). The tensions between the Soviet Union and US after World War II lead to the creation of the Berlin Wall, which stopped citizens from East Berlin from fleeing to west Berlin.
They moved there because water and roads were the methods of transportation, which was something they needed for trade. It was beneficial because it made trading goods easier.
