Structured output makes model responses easier for software to validate and consume. A response might contain a category, confidence value and short explanation, each with an expected type. Some APIs enforce a schema at generation time; others rely on prompting and validation after the response arrives.
For example, an inbox triage tool might require a category from a fixed list, a priority from 1 to 3 and a one-sentence reason. Because the shape is known, a workflow can route the message without parsing free text. Keep schemas small and descriptive: fewer fields with clear names tend to be easier for a model to fill correctly than a large nested object. The same idea applies to a tool schema.
Even when a provider offers schema enforcement, validate business rules and handle missing or unusable values. Correct JSON does not guarantee that the information inside it is true. Related: JSON Mode, Output Parser.