JSON is strict about quotation marks, commas, braces and data types. A single missing comma or closing brace can make an otherwise useful response invalid. Formatting JSON with indentation makes the structure easier to inspect.
Validation is especially useful when JSON comes from an API, configuration file or generated output. If validation fails, read the reported location first and then inspect the surrounding property or array. Remember that a line number can point near the error rather than exactly at its cause.
Do not paste private credentials, access tokens or other secrets into a public service unless you understand how that service processes the input. Browser-side processing is preferable for sensitive development snippets when practical.