Strong parameters answer only which keys may pass. A Permittable contract also says what each field should be: it casts the value to a declared type, validates bounds and formats, applies defaults, and renders every failure as a 422 that names the offending parameter. Because a contract is class-level data rather than code inside the action, it can also be checked against the database when the controller loads, so a column dropped by a migration fails the deploy instead of the request. activesupport is the only runtime dependency.