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