Why does the US president need a "designated successor"?

If the President of the United States is killed, incapacitated, or removed from office, the Vice President becomes the acting President. If the vice president is also unable to assume office for similar reasons, the right passes to the Speaker of the House of Representatives, and there are a total of 18 people on the presidential line of succession, including the Secretary of State, the Attorney General, and all cabinet members. When all of them are required to gather together with the president, for example, during his annual address to Congress or inauguration, a successor on duty, or, literally, "designated to survive," is selected from this list. He is placed in a physically remote and secure location, and if a disaster or terrorist attack occurs that kills all the top leaders, the designated successor becomes acting president and prevents chaos from ensuing.

The practice of designated successors originated during the Cold War due to the risk of nuclear attack. Only members of the cabinet who are eligible to be elected president (citizens over the age of 35 who have lived in the United States for at least 14 years) can be selected as designated successors.