Apple is saying this: "we need to scan your phone because we need to look for this material, and by the way, we're only scanning things _we've been scanning for years already_"
Basiclaly, everything else aside, in the current state of things, this feature makes no sense. They are not scanning anything they weren't scanning already (literally, everything on iCloud) so why did they bother?
A bunch of people are making speculative claims that Apple is doing this to enable encryption of data in iCloud, but there's no reason to believe that's true. Apple certainly hasn't mentioned it.
So .. why?