Not sure if you are using several vaults to organize different areas of your life.
In my case I only have one vault where all my notes live.
I use folders to organize different projects inside my vault.
Of course with the use of #tags and [[links]] one could argue that folders are not needed. I’ve definitely made that argument before. However an organized and long term functional system without any folders is something really advanced and even unconvinent sometimes.
My use of folders examples
Projects
if I feel the need for it I create a new folder for a new project (example a book I’m writing has it’s own folder)
Obsidian templates
Obsidian templates can be nicely organized in a folder
Integration with Other apps
I use other apps to read my markdown files, I have a folder inside my obsidian vault for this when needed, specially if the app deals with markdown files in a different way to obsidian.
I still like having such a filder inside of obsidian for better integration
I write my website from markdown files
This is more advanced, but still a great use of folders. I use Gatsby to create my website from markdown files. Having one folder set for this works perfectly
There are way more uses for folders and combined with #tags and [[links]] there’s tons of excellent ways to create a great system.