The original system code paired up with the ID ensures the uniqueness of each record.
Oracle permits you to import data from multiple systems and so there is a slight chance that you could have duplicates if you didn't have an original system code.
You can use these pairs to identify records in many import objects including customer, contact, and consumer.
Including these pairs in your file is one alternative for identifying records so you can update them later or import related information.
After you import an object for the first time, you can reference it using an internal ID such as PartyID or LocationID, but you'll have to do more work to get these.