> Why is reading plain text out of /dev/mouse and writing plain text to /dev/draw an improvement over how Linux or OS X or Windows does it?

It's very useful because uou don't need a C FFI to access OS functionality, graphics libraries and the like. Any language whose implementation allows you to read and write files can already do a lot of things (including GUIs, IPC and so on). There are very few wheels to reinvent. Furthermore, any program that wants to integrate with yours also needs to do nothing other than open, read, write and close files. You can expose any function or configuration variable of your programs through files -- and all you need to do for that is to read and write byte or UTF-8 streams.

Things go a bit further than just "IPC is done by reading and writing files". It's only one of the principles behind Plan 9's approach to files and filesystems.