What did the nazis primarily use concentration camps for during world war ii?

They used them to keep the people who they captured. They captured these people because they either were or looked like Jews. These people were put under harsh conditions going to the concentration camps. They also kept prisoners of war in there. Obviously, nobody wanted to go there. If you want to know more look up Anne-Frank's story. She wrote everything she lived during the time Hitler ruled. She died though and didn't make it out alive. She also went to a concentration camp if I remember correctly.

Hope this helps!