At the moment, I suspect the designers wanted to offer a tree structure way of accessing your images. Neither the Picasa web interface nor Google Photos nor Picasa desktop client allow you to see a tree structured organisation of your image files.
b. images from posts not in albums
c. autobackup images from phone or computer (guess)
d. images from other sources like hangouts (guess)
e. showing only the currently chosen version (there might be more than one version)
f. properties like date/time stamp and you could choose a timeline view.
You can create virtual copies of an image file between albums.
Google Drive1. A default hierarchy tree of folders, in year/month naming e.g. Google Photos/2015/03
2. Every image file you produced from all the sources in that month is put in that month folder.
3. You can optionally see and find image files and move / make a different additional hierarchy within that base folder without breaking your G+ text post.
4. You can move image files out of that base folder into one of your own self made folders outside of the base Google Photos folder (does this count as a delete from G+ ? Not sure.
5. You can edit any of those image files with third party photo apps in the Drive menu without affecting your G+ Photos image and G+ text post. There will be Drive multiple versioning of your image file, not just the G+ Undo version.
In other words, you can carry out Drive actions and features that G+ Photos and Picasa Web webapps lack, all without breaking your G+ text posts, except image file deletion.