Searchlights are only of any use in missions where the rules for night fighting are being used, such as the Night Fight mission. They allow one enemy unit spotted by the vehicle or Dreadnought to be fired at by any other Ork units that are in range and have a line of fire (the enemy unit has been illuminated by the vehicle's searchlight).
However, a vehicle or Dreadnought that uses a searchlight can be fired on by any enemy units in their next turn, as they can see the searchlight shining out into the darkness.