It prevents failures when JSON contains extra fields not present in Java classes.
Setting FAIL_ON_UNKNOWN_PROPERTIES to false allows your application to skip over any unexpected fields in the JSON, making your code more resilient to changes in the JSON structure. For example, if a shipping API introduces a new field in its JSON response, your code won't break if this setting is configured.
Additional Notes
Why configure FAIL_ON_UNKNOWN_PROPERTIES to false?