A Social Security number is probably the single most used piece of identifying information in the United States, and there are several places you can find your SSN. It is used for identification in everything from banking and financial transactions to applying for retirement or disability benefits. You use your SSN to file tax returns, apply for loans or credit cards, and track your employment history. Without a valid SSN, you would have a difficult time living in today's world. Your employer will also need your Social Security number to track your work credits and pay your Medicare and Social Security taxes.