See, since Google Drive is integrated into the Files app on Chromebooks, with automatic syncing turned on, you could access and edit/upload Google Photos images directly from the Google Photos folder in the Files app.
According to Google, this change is created to prevent the accidental deletion of items across products. The automatic syncing that happens between the two may be convenient for many but it's also caused some confusion among users who, for instance, might have deleted some photos from their Photos app only to discover later that they're gone from their Drive as well.
Apparently users have been confused that deleting a photo in Google Photos deletes the copy of it in Google Drive - and vice versa - and other oddities such as edits made in Google Photos not showing up on the photo in Google Drive. But the Google Photos app already has a share function that can let you send pictures to Google Drive and other third-party services.
"Any photos or videos from Drive in Photos that you have uploaded prior to this change will remain in Photos", added Google. In contrast, all the new photos and videos that you upload either on Google Drive or store in Photos won't be available across both products. In short, they confirm changes that were predicted last month following discovery of references to the change in updated APKs - Google Photos and Google Drive will no longer share your photos between them. So when you delete one in Drive, the original in Photos will remain.
The confusing thing with the new system is that photos that are uploaded in original quality to both services will now count towards your storage quota twice.
Google also says that Backup and Sync for Windows and Mac will continue to work, but that if you upload in original quality using that service it'll only count towards storage once for both services, because that's not at all confusing. The same thing applies in the opposite as well - new photos/videos in Photos won't automatically appears in a special folder in Drive.