One of the reasons was to keep track of the victims and their nutrition. In the early liberated camps, victims would die from eating too much food. So we started keeping much better records of what the camp residents were eating. Unfortunately, the best way to do this was to keep the survivors in one place, and that was initially the pre-existing camps.

http://www.yadvashem.org/yv/en/education/lesson_plans/bergen_belsen.asp

A quick note on your terminology: "death camp" is used to refer to one of the five extermination centers (the preferred term rather than "death camp" since death was a feature of the whole camp system), which were Auschwitz-Birkenau, Bełzec, Chelmno, Sobibor, and Treblinka (Majdanek is also sometimes included in this category as well). Mauthausen was a legendarily brutal concentration camp, but it should not be referred to as a "death camp."