Off the top of my head: DoS, providing perverts with confirmation that an image is what they want it to be, giving organized groups intel that images are known to law enforcement, etc.

Perhaps to avoid people figuring out how to evade it?

Just compressing the images into an archive, if not encrypting them, is enough to evade such a filter.

There are a lot more legitimate uses for a public repository of CP hashes along with free software for verifying them locally. Not only entrepreneurs and online community operators who don't want the stuff on display, but also users of poorly moderated online communities who don't want the stuff in their browser caches.