Honestly, I'd argue that few people do. At a previous employer, we used it in situations where would have been much better off with a more mainstream language, in my opinion.

Of course, there are few situations that require any particular language.

Of course. But for example if you are writing a compiler, an interpreter, or anything that has to manipulate something that resembles a structured tree of meaningful data, OCaml is particularly well suited :).

Any language with ADTs and pattern matching is reasonably suited for that task, and unlike 20 years ago, these aren't obscure features anymore, so there are many options.