Why is Google Photos Sync Read Only
Google Photos Sync is a backup tool only. It never makes any changes to your Google Photos Library in the cloud. There are two primary reasons for this:
The Photos API provided by Google is far too restricted to make changes in any meaningful way. For example
there is no delete function
you cannot add photos to an album unless it was created by the same application that is trying to add photos
Even if the API allowed it, this would be a very hard problem, because it is often hard to identify if a local photo or video matches one in the cloud. Besides this, I would not want the responsibility of potentially trashing someone’s photo collection.