I pulled this from wikipedia, The last part is the most important. There is information that in the wrong hands can hurt us as a nation. Also the people who gather that information are put in harms way. Maybe you would understand this way. your friend tells you a secret about someone that they shouldnt of told you. You go back to that person and say "hey guess what I heard about you." The person now knows that the original friend broke their trust and might want to beat the shit out of them. thats how it harms people.
The desired degree of secrecy about such information is known as its sensitivity. Sensitivity is based upon a calculation of the damage to national security that the release of the information would cause. The United States has three levels of classification: confidential, secret, and top secret. Each level of classification indicates an increasing degree of sensitivity. Thus, if one holds a top-secret security clearance, one is allowed to handle information up to the level of top secret, including secret and confidential information. If one holds a secret clearance, one may not then handle top-secret information, but may handle secret and confidential classified information.
By law, information may not be classified merely because it would be embarrassing or to cover illegal activity; information may only be classified to protect national-security objectives.