I have seen some questions related to functional programming on stackexchange sites which suggests it has significant popularity. I have some experience with it from many years ago in Lisp/Scheme and have seen commentary on the difference between functional and imperative/mutable approaches eg .
The wikipedia page on the subject says "Pure functional programming disallows side effects completely and so provides referential transparency, which makes it easier to verify, optimize, and parallelize programs, and easier to write automated tools to perform those tasks." But it is known to have a cost of up to a factor of $O(\log(n))$ slowdown factor  due to lack of mutable data structures, leading to a lot of research in functional programming datastructures that have a much different flavor than mutable approaches .
Along these lines,
What is an example for which the purely functional programming approach leads to better overall results than the imperative approach?
(Also it seems the word "mutable" here is a bit subtle. functional languages appear to allow one to "store" data eg via balanced trees, but not later "change" it. This is a little different than mutability eg in java, where initial storage and later changing are nearly identical operations eg with "setter" methods on the Java objects.)
 functional programming, wikipedia
 stackexchange, efficiency of purely functional programming
 tcs.se, What's new in purely functional data structures since Okasaki?
 Structure and interpretation of computer programs, by Abelson & Sussman