From Command and Conquer Wiki

Due to heavy bombardment from the Soviets the Allies developed fake buildings which were made to decive enemies into targeting them to draw attention away from their real counterparts. The Fake War Factory was the most common fake buildings used because of the high priority the Soviets (and most enemies) have to destroying War Factories. With just a few Fake War Factories up it would be much more difficult for Soviet MiG-29s to destroy the real War Factory. IT was not used by Nod or GDI.