This API is used to understand what additional inputs are required when the requiresAdditionalInputs property in the response to the getRates API indicates. This is a required step for international shipments. The response returns the schema you must use to provide the additional inputs when you call the purchaseShipment API.
You can also use the information in the schema to inform your building of dynamic forms for your end user to use to provide the additional information required.
Additional Input API Reference has further details.