[mumblemumble]

Its official use case is hacking recursion into a language that doesn't support recursion, but does have first-class functions. To the best of my knowledge, the only language that fits this description is the lambda calculus itself.

There is a paper, Y in Practical Programs ( https://blog.klipse.tech/assets/y-in-practical-programs.pdf) that presents some interesting things you can layer on top of the technique. I'll leave it up to others to debate whether doing so is genuinely practical, or more of a game of functional code golf.