Every successful HTTP API response should possess a JSON Schema reference that defines and allows for the validation of responses, except for when updating with a PUT, or deleting with a DELETE, and some other edge cases. Providing schema for HTTP responses allow for further validation and serialization of responses in gateways, applications, and integrations.
Have schema well-defined for successful HTTP API responses contributes to quality, reliability, and security, keeping API consumption a known activity. Response schema for 2xx API responses are essential for almost every API response you will design, just make sure they are as standardized as possible, and always articulated as robust JSON Schema artifacts.