Trade was the motivating factor. The silk road accross Asia was besieged by bandits & required bribes to a number of nations to transport goods to & from Asia. Therefore a sea route would be a much safer & less expensive means of getting trade goods to & from Asia.
As the head of the administration of his province, the satrap collected taxes and was the supreme judicial authority; he was responsible for internal security and raised and maintained an army.They served as the chief judge for their region, adjudicating disputes and decreeing the punishments for various crimes. Satraps also collected taxes, appointed and removed local officials, and policed the roads and public spaces.
