It's a stupid system. Technically only the social security administration itself is allowed to require an SSN. But most banks, companies, etc. do require an SSN from you, or else they put you through a ton of hassle. In practice almost everyone just gives his SSN, and the few who don't can be subjected to more scrutiny. SSNs are also reused after you die. After all, we don't want to run out of numbers, our nation's greatest resource.