Searchlights were first used in World War I to create "artificial moonlight" to enhance opportunities for night attacks, a practice which continued in World War II. Artificial moonlight was invented by historian and tank warfare theorist, Gen. J.F.C. Fuller.
Searchlights were used extensively for defence against nighttime aerial bomber raids around the time of World War II. As radar was a brand-new technology used only by the British for early warning purposes, anti-aircraft flak cannons required visual targetting; searchlights were used to illuminate aircraft in the sky so that the gunners could see them.
More recently, searchlights are often used in advertising, for instance by automobile dealers; the beam of light is visible over a large area, and (at least in theory) interested persons can find the dealer or store by following the beam to its source. This also used to be done for movie premieres; the waving searchlight beams are still to be seen as a design element in the logo of the 20th Century Fox movie studio.