An alternate use case would be looking at scope or the “Why not both?” approach. When I use the word scope, I use it mainly in a programming context where you have a local and global scope. Although obsidian handles this a bit different, it could work quite well.
Lets say you have a large set of interconnecting ideas, but distinct projects. You have your journals, books you read and the notes you took, random factoids you found at 3am on wikipedia, over 9,000 different references to tv-tropes, etc. You get the idea 
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Then, for example, lets say you are working on some writing like a novel. You would have a folder for writing projects under which you would have a folder for that book. Inside that folder you would have a typical structure with folders and files for drafts of each chapter/section as well as files for notes/research such as a “characters”, “settings”, or “Timelines”.
Now you might want to link and discover ideas between this novel and ideas you have else where. You are writing a character and want to reference your notes on the [[hero’s journey]]? No problem. Doing some world building and want to reference that paper you wrote on world politics two years ago. Great, all in the same vault.
Later, however, you want to focus in on your drafts and do edits. Avoiding distractions from all the tags, preventing yourself from getting lost taking a [[wiki walk]], and mitigating the distraction of the cool graph of all the linking objects of your work, are all needed.
Again, no problem. Just open the folder for the novel as it’s own vault and edit it there. You could even have different settings and CSS. Fewer panes, less distracting colours, whatever you need. Then later, you can pop back out to the main vault and you are back into the global vault.