In my years living in the United States, I've used my SSN several times, but I've never been required to use my Social Security Card. Is there any process or activity that requires the card that your Social Security Number is printed on?
They can be used for ID. For government purposes various type of cards have points and a certain number of points is needed for various purposes. A social security card can be used as such.
For example, when we go to get a drivers license, you can bring an ATM Card, Passport, Utility Bill, SS Card (as examples) for ID. Each of those has a different value (passport high, utility bill low). They add the values you present and they have to be above a threshold.