Yeah I think that was something Andy Wingo added over a decade ago that hasn't seen much love since, but along with elisp it's another good example of the compiler tower. If you write a language frontend that lowers to Tree-IL then you get to ride the compiler train all the way to bytecode for free. I think that's pretty neat.

Another interesting use-case for the compiler tower is writing a language that compiles a machine readable specification to another language. guile-xcb compiles XCB's XML spec to Scheme:

https://github.com/mwitmer/guile-xcb/blob/master/language/xm...