The target of NCMEC's database is stopping the distribution, and hopefully encouragement of CSAM. Besides how the distribution can negatively impact the mental state of victims, distribution and more eyes on it might cause more people already on barely-legal parts of the internet (ie. qanon message boards) to learn about ways to obtain CSAM and become part of the market of buyers for such material. If you stop that, then the population of people looking for CSAM shrinks and makes it less economical for criminal enterprises to target children specifically for the production of the illegal material.