Telecommunications over long distances is a natural area for fiber optics because of the high bandwidth possible with a single fiber strand. Fibers are lighter than copper wire so that they can more easily be transported to the cable laying site and can often be more easily routed. Fiber cables are much more difficult than copper wire to tap, so in addition there is a security advantage.
LANs and WANs take advantage of the afore-mentioned advantages as well as noise immunity. Factory automation, in particular, utilizes the lack of sparks and noise immunity inherent with fiber.