Its useful to be able to use "contentMediaType" in contexts where binary media types are present. It does not make sense to use "type": "string" with binary data, because such data is not a JSON string no matter how much you try to map it into the data model.

A schema without a type can effectively be a hint not to worry about a detailed mapping. It is a way to use JSON Schema across media types without excluding binary content.

See OAI/OpenAPI-Specification#2200 for use cases.