Why did the Nazis convert the concentration camps into death camps?

Only one concentration camp was converted into a death camp, or rather it was expanded and a death camp was added, this was Auschwitz. The death camps that were had served their purpose, they were created to murder those Jews in the ghettos, when this was done, they were dismantled. There was still a need for a killing centre and with good rail connections and access to coal, Auschwitz was a good location.