Why should I register with the US Embassy/Consulate?
Registration at the U.S. Embassy or Consulate (in the country you are visiting) makes your presence and whereabouts known, in case it is necessary for a consular officer to contact you in an emergency. During a disaster overseas, American consular officers can assist in evacuation were that to become necessary. But they cannot assist you if they do not know where you are.

Registration is particularly important for those who plan to stay in a country longer than one month, or who will travel to:
*A country that is experiencing civil unrest, has an unstable political climate, or is undergoing a natural disaster, such as an earthquake or a hurricane.
*A country where there are no U.S. officials. In such cases, you should register at the U.S. embassy or consulate in an adjacent country, leave an itinerary with the Consular Section, ask about conditions in the country that you will visit and ask about the third country that may represent U.S. interests there.(http://travel.state.gov/travel/tips/registration/registration_1186.html)