Because you're searching for people, not documents.
For a normal law enforcement context, say you bust 1 drug dealer, and are trying to find more. Maybe they have an image of a "contract" (document outlining terms of a drug deal) on their phone. You could find the other parties to that contract by searching everyone's phones for that image.
For a national security context, you could imagine equivalents to the above, you could also imagine searching for classified documents that are believed to have been leaked, or maybe searching for documents you stole from an adversary that you believe there spies are likely to be storing on their phones.
I'm saying documents here instead of images, but plenty of documents are just images, and I have little doubt that they could get this program to expand to include "scan PDFs, because you could make a CSAM into a pdf" (if it doesn't already?).
For a normal law enforcement context, say you bust 1 drug dealer, and are trying to find more. Maybe they have an image of a "contract" (document outlining terms of a drug deal) on their phone. You could find the other parties to that contract by searching everyone's phones for that image.
For a national security context, you could imagine equivalents to the above, you could also imagine searching for classified documents that are believed to have been leaked, or maybe searching for documents you stole from an adversary that you believe there spies are likely to be storing on their phones.
I'm saying documents here instead of images, but plenty of documents are just images, and I have little doubt that they could get this program to expand to include "scan PDFs, because you could make a CSAM into a pdf" (if it doesn't already?).