The primary use of the SSN is, as mentioned above, the account number of a person's Social Security account. However, because the number is unique to the issuee, it is often requested by various professional services, such as doctors' offices or financial institutions. It is also often requested by academic institutions. In fact, one community college I went to used the SSN as the student's college identification number.
Protecting your Social Security Number is important, because it allows access to your Social Security account. It also allows the SSA to uniquely identify the holder. Therefore, the card that is issued to you by the Social Security Administration should be kept in a safe place such as a strongbox. It should never be carried on your person unless it's been requested by an employer or a provider of legitimate services.
If someone requests your number, according to the SSA, "you should ask:
- Why your number is needed;
- How your number will be used;
- What happens if you refuse; and
- What law requires you to give your number." (http://www.ssa.gov/pubs/10002.html#protect)
(Much of the information here is referenced from the Social Security Administration's website at http://www.ssa.gov/.)