This is totally a biased cop out, but I'd say "most things".

Like, what's java's niche? I'd say it doesn't really have one anymore, it's just a generalist language. Ocaml is also a generalist language, but it has much better language paradigms, is safer, more concise, has more features with less baggage, etc. It's just good all around.

As a functional programming advocate, I'd point to safety, readability, and development velocity as features, but those are all around good and desirable, not really a niche.

I wish more companies were using languages like ocaml/f#/clojure vs c++/python/java when there is no specific reason to use the former.