> While I don't mind the toy problems, all those toy problems have something in common...

> They can be easily solved with recursion.

As a sibling mentions, recursion is pretty much required in pure functional programming. There is no notion of time or sequencing: everything we write is a definition. If we don't call another function, then our program will finish; if we don't use recursion (either directly, indirectly via functions like `map`, or via a chain of mutually-recursive definitions) then our program's behaviour will be fixed by the structure of our code (e.g. the number of calls we write). That's fine for simple calculations, but most useful programs depend on the structure of their data (e.g. processing lines in a file, etc.); that requires recursion, since we can't hard-code the right number of calls ahead of time.