Maintaning an empire is a hell of a lot harder than most people think it is. Roman emperors quickly discovered that if you are in Spain when Egypt gets out of line, you need to make sure the guy you need is not currently building a stupid wall in Britain. And even if he is, he’s probably not checking his emails regularly. So to get communications, troops, goods, food, trade, and taxes around as quickly as possible, the Romans built large networks of roads.
Octavian and the early emperors were the first to start the road building tradition. (Personally I think Octavian was just tired of walking through the mud after all his campaigning–anything for dry feet!) People quickly realized what a great idea this was, and imitation soon spread even to lesser officials in the provinces. You knew you had it made in politics if you had a road or two under your belt. Of course, you didn’t want just any slacker finding a goat trail and calling it a road, so a simple definition was soon made: if it was wide enough for a wheeled vehicle it was a road (paving optional).
Eventually the network spanned over 400,000 km of roads, and around 80,000 km was paved. All this practice made the Romans quite good at road building–their later paved roads allowed for the drainage of water so the streets didn’t turn into rivers (and anyone who’s ever played Oregon Trail knows that doesn’t turn out well.)
The army found the roads incredibly useful for ease of transportation to and from headbashings. And near the end, the invading barbarians found the roads even more useful as an easy stroll straight into the heartland. Which just goes to show you, the destruction of Rome was paved with good intentions.