U.S.A. Social Security number, and it's purpose. A quick generalization.
The Social Security Number is a unique number given to a person who is considered to be a US citizen. It is a retirement system, you agree to give a portion of your wealth so that your grandparents generation can retire, in exchange for the same deal when you are ready to retire.
Today the SS# is essentially a national citizen ID#, especially for government forms such as tax forms. Without one it is difficult to exercise one's freedoms. Any individual who can recite your SS# is considered to be you, with little or no other proofs of identity needed. SS#'s are the among greatest risk of identity theft, aside from Credit Cards. The rub is that many or most important transactions require a SS# as proof of identity, which means that you will be handing this number out to a lot of people you've never met before. People are asked for it so often, that few stop to think "Does this business really need my SS#?"
This is often used as a secondary form of ID, as the state issued ID and State ID# (Drivers License) are the primary form of ID. The State ID# used to be the SS# in my state.