Because SSN are the main way that banks and credit agencies use to identify people (which is not at all what they were intended for originally). If you know so

Because SSN are the main way that banks and credit agencies use to identify people (which is not at all what they were intended for originally). If you know somebody else's SSN plus a couple of semi public details (name, date of birth, address), you can go to a bank and borrow money under their name.