Parsers/interpreters/compilers/static analyzers. It's solidly established here. The Web Assembly reference implementation is written in OCaml for example.
> "why not just use go/node/racket/haskell/fsharp/rust/zig/etc"
I think Python, Java, C# or even PHP will come up before F#, Rust, Zig, Racket and Haskell. As for the answer, you can just use the same language pretty much everywhere. I think OCaml is probably the most versatile language of the lot. You can program in any way you want, target pretty much anything you want. How important is it, to be able to use the same language everywhere? I honestly don't know.
> when you are on a timeline and you aren't Jane Street...
I'm not sure what you mean exactly with "on a timeline", I guess it's a reference to the lack of libraries/documentation? If that's the case, I don't know how much it matters once you're experienced with the language. I'm not saying libraries don't matter at all, that would be silly. You don't want to do JSON parsing or a HTTP server yourself. But for small stuff, you can just write a bit more code that you would usually do. It's going to be way smaller than a full library, and it's going to be easier to use since you don't have to learn a new library. This is however just me thinking about it, I don't have any hard data on this, and would love to know if some exists.