The built-in check digits let a sender's bank validate the number before money moves, which cuts failed and misdirected transfers. Length varies by country, so a valid IBAN in one market looks quite different in another.
The United States does not use IBANs domestically — US accounts are identified by a routing number and account number. Merchants settling across regions therefore hold different identifiers per market, which is why multi-currency setups collect them separately.