The status line is the headline, the body is the small print; tools read headlines.

Clients, proxies, retry logic and monitoring dashboards all act on the status code before they ever parse a body. If a failure only lives inside a JSON field, every one of them thinks the call worked, and error-rate alerts stay silent. Put the outcome in the status, and use the body to explain the details.

Read it once, then a question tests whether the trick stuck.