Hi @Eccentric-J,
Allow me to describe my understanding of the original intended purposes; I will let others and project members to correct me or add to my description.
I use the backlinks with [[file:path/to/file][description]] in the same way you use. Using “file-refs” with the #+roam_key: property is an additional way of establishing backlinks.
There are two ways to use #+roam_key:. One is as you intend to use it: for a website. Perhaps an article or blog you find useful for you; you quickly capture the URL, and make a note to summarise its content (your first example with “Understanding file refs”).
Then later, you may have additional notes that refer to the same website. These additional notes should appear in “Ref Backlink” of the original note with #+roam_key.
Illustrating with a screen shot below, Area 1 is the original note for http://duckduckgo.com summarizing what it does (my sample has no summary… I appreciate the power of your imagination).
[EDIT: there was a mention of a bug, which has been fixed. The screen shot below has been updated to reflect the fix]
Area 2 shows an additional note referring to the note in Area 1.
Area 3 is the backlink for the original note. As you can see the backlink appears in the “Ref Backlink” section.
This is one of the two usages.
The other usage is more for academic writing. You may take notes for a book or journal article, and keep bibliographic information in a reference manager (such as Zotero, etc.) – like the screen shot below.
Left: PDF document shown in PDF-Tools in Emacs. Right: Notes via Org-noter in Org Mode.
This is equivalent to the “original note” for a website in the example above. #+roam_key: is used as the ID of this note with using the cite:cite-key syntax of Org-ref (sync’ed with your reference manager).
Later you can refer to this note by using cite:cite-key, like the screen shot below – it shows the original note on the left, and its backlink on the right. You can see that #+ROAM_KEY: holds the ID “cite:Ostrom2019” (defined in my reference manager), and it is used to cite the paper in my other note (shown as a backlink).
This is my understanding of the two “standard” usages of “File-ref” with the #+roam_key property. Hope this helps. Correction, comments are welcome. 