A generous explanation would be that they intend to add end-to-end encryption to iCloud eventually, together with this new scanning technology as a "government backdoor". A less generous explanation would be that they intend to expand this scanning to non-iCloud media eventually.
They also confirmed it is only when the photos upload to iCloud: https://www.macrumors.com/2021/08/05/apple-csam-detection-di...