> I don't think I've ever seen an argument for why "everything is a file" is a desirable thing.
A file system is a tree of named objects. These objects are seamlessly part of the OS and served by a program or kernel driver called a file server which can then be shared over a network. Security is then handled by file permissions so authentication is native through the system and not bolted on. It fits together very well and removes so much pointless code and mechanisms.
A great example is a old uni demo where a system was built to control X10 outlets and switches (early home automation gear). Each device was a file in a directory tree that represents a building with sub directories for floors and rooms - e.g. 'cat /mnt/admin-building/2fl/rm201/lights' would return 'on' or 'off' (maybe it's a dimmer and its 0-255, or an r,g,b, value or w/e, sky's the limit, just put the logic in the fs). To change the state of the lights you just echo off >/mnt/admin-building/2fl/rm201/lights.
Now you can make a script that shuts all the lights off in the building by walking directories and looking for "lights" then writing off to those files. Maybe it's a stage, all your lights are on DMX and you like the current settings so then you 'tar -c /mnt/auditorium/stage/lighting|gzip >student_orientation_lighting_preset.tar.gz' and do the reverse over-writing all the archived settings back to their respective files. You could even serve those files over smb to a windows machine and turn lights on and off using notepad or whatever. And the file data doesn't have to be text, it could be binary too. It's just that for some things like the state of lights, temperature or w/e can easily be stored and retrieved as human readable text.
That is the beauty and power of 9p - its removes protocol barriers and hands you named objects seamlessly integrated into your OS which you can read/write using regular every day tools. It's a shame so many people can't grasp it.