In April 1945 why did the death march take place?

There were several events that were called "death march". The most famous one was the Bataan Death March in 1942 on the Phillippines. Most of the death marches occurred at Prisoner of War camps in Germany. The Germans kept moving the prisoners out of reach of the advancing Russians. In late January 1945, the Stalag Luft IV in eastern Prussia evacuated approximately 6000 prisoners from the camp within sound of the Russian artillery. On 2 Feb 1945, about 4000 POWs were marched out of Stalag VIIIB(aka Stalag 344). I'm not an expert on the concentration camps but some of them may have moved prisoners. However, their main objective was to elminate the evidences of those who had died.