When I’m navigating the music on my usb drive I get folders such as “artwork” which contain only .jpgs. I only use Kodi for listening to music, so it would be great if I had the option to only show .flac/.mp3 files to select, and only show folders which contain .flac/.mp3 files within them (directly, or in sub-sub folders).
You should use the library mode and the smart filters / source filters.
There’s a tons more file extension than that not talking about addons that can be browsed without extensions. And Kodi does not offer such function so filtering subdirectories would require to get all the data and filter on client side, it would be insanely slow and inefficient.