Interesting. Is this just literally “fun”, or do you see real world use cases?

I have used this in the past when building shell scripts and Makefiles to orchestrate an existing build system:

https://github.com/jez/symbol/blob/master/scaffold/symbol#L1...

The existing build system I did not have control over, and would produce output on stdout/stderr. I wanted my build scripts to be able to only show the output from the build system if building failed (and there might have been multiple build system invocations leading to that failure). I also wanted the second level to be able to log progress messages that were shown to the user immediately on stdout.

    Level 1: create fd=3, capture fd 1/2 (done in one place at the top-level)
    Level 2: log progress messages to fd=3 so the user knows what's happening
    Level 3: original build system, will log to fd 1/2, but will be captured

Red hat and other RPM based distributions recommended kickstart scripts use tty3 using a similar method

Multiple levels of logging, all of which you want to capture but not all in the same place.