original: They were built mainly by nations like Germany during World War II to protect important industries from aerial bombardment.
simplifications: They were built by nations like Germany during World War II.  The purpose was to protect important industries from aerial bombardment. They were mostly made by nations like Germany during WWII to protect against aerial bombings. They were built by nations like Germany during World War II to protect important industries from air attacks. They were built by nations like Germany to protect important industries from aerial bombardment. Nations like Germany built them to protect important industries from being bombed during WWII. They were built by nations like Germany during World War II to protect industries from aerial attacks. They were built by countries like Germany during World War II to protect important industries from bombs. Built mainly by countries like Germany during WW II, they saved major businesses from bombings. During World War II, nations like Germany built them to protect important businesses from air bombing. Germany protecting major industries from aerial bombing during World War II.