The original purpose of the Social Security number was to enable the Social Security Board to maintain accurate records of the earnings of individuals who worked in jobs covered under Social Security. Even though its use has expanded, the only legal requirements to give your SSN are for government purposes. Other purposes are not illegal, but are not mandatory.
Who has the right to request your SSN? Federal law says state Departments of Motor Vehicles, tax authorities, welfare offices and other governmental agencies may request your SSN as proof that you are who you claim to be. However, no one can deny you a government service or benefit for failing to provide that number unless federal law specifically requires it.
You aren't legally required to provide your SSN to businesses unless you are: engaging in a transaction that requires notification to the Internal Revenue Service; initiating a financial transaction subject to federal Customer Identification Program rules; getting Insurance; applying for credit or a loan; or conducting business with any company that sells products or services that require notification to the IRS.
For example a lot of businesses, such as doctor and dentist offices, request you and your spouse’s SSNs, along with other personal information. Unless your health or dental insurance uses your Social Security number to identify you, your doctor or dentist do not legally need it. HIPPA does not require the use of SSNs. If you have received your new Medicare card, it would not require your SSN either.
There will be places that insist that it is a legal requirement. In those instances, ask to speak to a manager. If you refuse to provide your SSN, they may choose not to do business with you, but there's no law that prevents them from asking for it.
It's important to remember that once a company has your SSN or other personal information, there are few restrictions on what they can do with it. So you'll to want share this information only when absolutely necessary or required by law. The best advice is to give the minimum amount of information to get the service you desire. This would include non-medical demographic information.
Just because you are asked for your information doesn’t mean there is a legal or compelling reason for the request. Numerous corporations and other entities are hacked every year. It is up to you to protect your information.