Google Photos and most photo backup solutions work simply by uploading all images you’ve authorised access to to the cloud.
In the case of Google Photos, that is Google’s cloud which is tied to your Google account. But what if you then go ahead and delete the same photos that have already been uploaded from your device? What happens? So far, on Google Photos, nothing.
But that is changing. Going forward (for Android users, version 1.23 and up), the app will offer to delete images that have been deleted on local storage from the permanent cloud backup as well.
For some of us, local deletions of images are made so as to save on valuable limited storage and it does not necessarily mean that we also want the images we have discarded locally to also be banished from our online lockers. Which is why Google’s approach, that of asking users all the time, is better than what Apple offers with iCloud Photo Library which simply goes ahead and deletes everything done away with locally.