It recovers duty on domestically sourced exports
A company that sources domestically and exports cannot put those goods in a zone to save duty, there was no import duty on them to defer in the first place. But under substitution drawback, it can match those exports against duty-paid imports of the same 8-digit HTS and recover duty it could never have reached through an FTZ. That is recovery an FTZ structurally leaves on the table.
