For "field" applications, particularly in the military, defence and security, aeronautics and aerospace sectors, embedded systems are often subjected to very severe environmental constraints that a conventional industrial PC such as a "PC Box" cannot meet.
Indeed, these extreme environments with, in particular, problems of extended temperatures (start-up & operation), high levels of shocks and vibrations, dust, humidity, salt spray, etc. often require much more resistant and reliable solutions that are commonly called "rugged embedded computer".