Why do the governments make people evacuate?

During World War II, many persons were evacuated from British cities and towns, even from small villages and rural areas, for several reasons. One reason was the danger involved in allowing the evacuees to remain in areas that were subject to enemy attack. Another reason was the need for security: troop movements and outposts, as well as military installations and training grounds, often needed to be kept secret from any potential spies or "loose-lipped" citizens.