It is not at all uncommon to want a spec for an api that’s out of your control. Or to want to define shared data structures at the transport layer which may have “owners” in heterogenous languages. Spec languages are (well, can be—so very many are terrible) very nice in those cases, which are not at all rare. They may well be more common than the “we’re writing a new single-language monolith that is free to control the shape of the data it slings” use case.

for internally consumed APIs I somewhat agree, but if you wanna expose APIs to external developers it usually does pay off to spend a bit of time on the API design itself, what APIs would you expect to see if you were the user of the your own service - and this is where tools like this really help imo - you could even preview your API in some openapi doc tool like readme or similar, in which case it's like previewing your product before releasing it