Why did they make concentration camps for the Jews?

Answer 1

1. For your general point, please see the related question.
2. The early concentration camps were for political opponents.
3. Contrary to a widespread misconception the decision to kill all Jews in areas under German control was not taken till 1941. (Jews in Germany and some occupied countries were allowed to leave if they have somewhere to go to and could actually get there). There wasn't a long-standing plan to commit genocide.

Answer 2

So the people of Europe and Germany in special would not know what was happening to them. The Nazi's made everybody believe the Jews would be send to working camps, but we know better now. A few camps truly were working camps with humane conditions, however they were only used to make Propaganda movies. These movies would make the people believe that nothing bad was happening to the Jews and also even worse, would make Jews turn themselves in to Nazi-search parties because they thought the conditions weren't that bad after all.

A final reason is the effect on the German army itself. Before the camps German soldiers had to mass execute the Jews. Ofcourse this would lead to trauma and demoralisation, effects the Nazi's didn't want.