Why were people forced to go on "death marches"?

Near the end of the war, when Germany's military force was collapsing, the Allied armies closed in on the Nazi concentration camps. The Soviets approached from the east, and the British, French, and Americans from the west. The Germans began frantically to move the prisoners out of the camps near the front and take them to be used as forced laborers in camps inside Germany. Prisoners were first taken by train and then by foot on "death marches," as they became known.