Social Security Administration replies to inquiry on use of SSN by businesses.

Below is the e-mail reply I received.  The only thing missing is an incomplete laundry list of "situations" which "require an SSN".

Emphasis added.

from:SSA.Comments@ssa.gov
to:ken
date:Sat, Jun 20, 2015 at 11:18 AM

The Social Security number (SSN) was originally devised to keep an accurate record of each individual’s earnings and, subsequently, to monitor benefits paid under the Social Security program.  However, use of the SSN as a general identifier has grown to the point where it is the most commonly used and convenient identifier for all types of record keeping systems in the United States.
 
Specific laws require a person to provide his or her SSN for certain purposes...

The Privacy Act regulates the use of SSN’s by government agencies.  When a federal, state, or local government agency asks an individual to disclose his or her Social Security number, the Privacy Act requires the agency to inform the person of the following:
 
-- the statutory or other authority for requesting the information;
 
-- whether disclosure is mandatory or voluntary;
 
-- what uses will be made of the information;
 
-- the consequences, if any, of failure to provide the information
 
If a business or other enterprise asks you for your SSN, you can refuse to give it.  However, that may mean doing without the purchase or service for which your number was requested.  For example, utility companies and other services ask for a Social Security number, but do not need it; they can do a credit check or identify the person in their records by alternative means. 
 
Giving your number is voluntary, even when you are asked for the number directly.  If requested, you should ask why your number is needed, how your number will be used, what law requires you to give your number, and what the consequences are if you refuse.  The answers to these questions can help you decide if you want to give your Social Security number.  The decision is yours.
 
For detailed information, we recommend our publication, “Your Social Security Number and Card,” at the following Internet address:
 
http://www.socialsecurity.gov/pubs/10002.html
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