It's interesting as a historical study. Plan 9's a direct successor to Unix, and made by same people who would invent the Go programming language. The same philosophy behind Go and Unix is woven into Inferno/Plan 9.

Check the “introduction” paper about the family of OS [1], it provides a good introduction to get the how and why.

My two cents is that composing applications at the FS level is more expressive than Files/Sockets/HTTP requests.

Gluing up different of pieces of software becomes easier as most of the time it becomes just a matter of mounting a filesystem and you don’t need to worry about it being provided by a local or remote process.

[1] - http://doc.cat-v.org/plan_9/4th_edition/papers/9

In general, simple interface is easy for the programmer to understand, and provides less opportunity for system bugs and strange interactions.