It’s all about having a machine-readable definition of the API. When you have that, you can support strictly-, strongly-typed languages much better. In a language like C, Java or Rust, which would you rather: a JSON data type that is essentially Map<String, Object>, or a struct (or class, or whatever) that actually represents the fields and types that exist?

It’s useful to be able to quickly generate a client library with fair ergonomics for such languages, rather than having to use a weakly-typed, string-heavy general-purpose HTTP library. Languages like JavaScript, Ruby and Python don’t benefit so much from things like this, but even so there are definite advantages to it.