Certain Wikibase API modules, e.g. SetClaim or SetReference, do have a field that must contain a JSON blob. Even if these API modules are not made to be used manually by a user, we want users to play around with the API and learn how it works. That's one of the reasons why we have the ApiSandbox.
Problem: The fields to input JSON blobs are tiny, single-line <input> fields. Editing a JSON blob in this field is pretty much impossible.
Suggestion: Turn JSON input fields in the ApiSandbox into <textarea>s (depends on T98457).