It was creating for the purpose of tracking an individual's account by the Social Security Administration. It later became a de facto identifier and, even worse, is many times abused as a form of authentication, but it was never designed to be either.

As a result, we have processes that ask for or require a social security number that aren't even related to the purpose for which it was created: Health care, loans, debt collection.

Notably, some citizens of certain religious sects, like the Amish, do not have social security numbers.