The answer is right there in your question: labor camp. The Germans were at war and viewed all potential labor as a military resource. The Germans had competing requirements for war industry labor and military manpower. Making the Jews and other classes of people that were put into camps work freed up German men for military service. Goeth wasn’t sparing them, he was using them for the war effort. That he later murdered some of these workers on whim shows the depths people can sink to when they sufficiently dehumanize others, but his focus was still largely on getting maximum production out of his prisoners. Schindler used this practical imperative to save as many of the people as he could, arguing that he even needed the children because their small hands let them polish the insides of shell casings.