It's true that other empires of the ancient world built very impressive roadworks. We might point to the Neo-Assyrian royal road that stretched from Anatolia, through the major cities of Assyria such as Nineveh and Arbela, to Susa in modern-day southwest Iran. The Achaemenid Persian empire expanded on this road network as well, building roads to Babylon and Persepolis and all the way to Bactria and the Indus river. In and near cities these roads could be made from durable materials, like bricks and bitumen mortar, and some were even cut from rock like at Tang-e Bulakhi where a 1.7 metre wide road was cut 10 m into the rock of the gorge. However, parts of the Assyrian and Persian royal roads were probably cleared dirt track, kept wide and flat so that chariots could move over them but prone to muddying up in bad weather (there's an example of this happening in Xenophon's Anabasis, 1.5.7). Some of this work was done by soldiers; at Pieria, Xerxes assigned a third of the army to deforesting the area so that they could build a reliable road. Other sources hint at the use of wooden roads too, as Xenophon's Cyrus (6.2.36) calls for woodcutters, diggers, and shovellers to begin building a road.

In the Warring States period and in Qin and Han dynasty China, substantial networks of tamped-earth roads were built, often paved with stone. It's been suggested that part of the reason for the state of Qin's success was the road network they built in Sichuan, which enabled them to efficiently exploit their region's mineral and agricultural potential and impress more soldiers and labourers.

Greek authors spoke very highly of the Achaemenid postal system operating along the road (Herodotos 8.98, Diodorus Siculus, 19.17). Qin and Han dynasty China also had mounted relay couriers who could supposedly carry a message 1,000 li (around 420 km using the Han li) in twenty-four hours in an emergency. This brings up the other ways in which a road system empowers an empire; it enables more rapid and reliable communications. An empire can only work if it can respond to situations within its area of control within a reasonable timescale, and the shorter it takes messengers and troops to reach one region, the more control it can assert there. It's difficult to closely control a region if it takes you years to find out about an uprising there and years more to respond. You would be forced to give a lot of latitude to whoever governs there in your stead and you would have a lot less bargaining power against them.

The first major Roman roads make a lot of sense in this context. After the second Samnite War, the Romans had found their ally in Capua insufficiently obedient. The Via Appia, built in 312 BCE by Appius Claudius Caecus, enabled Roman troops to march to Capua in five days. Procopius (Wars, 5.14.6) noted that it was not originally paved. While the parts inside the city were paved soon after, most of its length was only paved in the late 2nd century BCE during the time of Gaius Gracchus. This was done as the Romans installed prefects there to oversee and limit the power of local elites; the two projects worked along similar purposes. A road can also be the means for colonisation and reclamation of territory, as well as the extraction of resources, such as in Roman Dacia and the pontes longi in Germania.

Road transport has often been secondary in the economies of ancient and medieval empires to water transport. Egypt had the Nile. Mesopotamia had the Tigris, Euphrates, and various canals. The ancient Greeks built some large and durable roads such as that from Sparta to Arcadia, Athens to Peiraieus (its port), Argos to Tegea, Athens to Eleusis, and Elis to Olympia. These could serve military, economic, or religious purposes, but none were all that long and trade between the cities of the peninsula (and with partners in Anatolia and further abroad) mostly travelled across the sea. The major lines of communication for the Athenian, Carthaginian, Venetian, and Ottoman empires were maritime ones, but even in medieval England goods and grain produced in the interior travelled downriver to export centres on major rivers or the coast. Water transport tends to be faster and have a much higher capacity, but obviously it only works where there are navigable waterways. However, an empire that has a lot of territories distant from large bodies of water will face a lot of difficulty controlling them without effective overland communications. Rome needed to control and exploit inland Iberia, Gaul, and Dacia, and defend borders in Germania, along the Danube, and in North Africa, which would have been impossible without rapid communications and durable roads for animals, vehicles, and troops. The roads that solved this problem also brought Roman goods, settlers, and culture to the edges of its empire.

Roman roadmaking certaintly didn't emerge in a vacuum, but they brought together and added to existing technologies they may have acquired from the Greeks, Etruscans, and Gauls. The Greeks used lime mortar; the Romans used lime mortar with volcanic pozzolans added and sometimes with gravel added too, making concrete. As they refined their design over time, they raised a high agger of layers of material topped by paving or metalling in the centre of the road flanked by drainage ditches, making the road more resilient to winter, freight, and water and also more defensible. The pavement on top of this agger could be a metre thick. Standard dimensions existed for these roads, probably set down in the 5th century Law of the Twelve Tables, mandating a minimum width of eight feet (so two wagons could pass each other) and sixteen feet at bends. They also built many stone bridges, some of which are still used today like the Pons Fabricius in Rome.

Roman roads are also unusual in their economic and social context. Polybius (6.13.3) reports that the largest part of the Roman state budget was for public works, and indeed huge sums were often assigned for the building of roads and aqueducts. Sometimes the burden was shared between the state and local landowners. For example, in 123/124 CE, 1,470,000 sesterces for the upkeep of the Via Appia came from the Emperor Hadrian and 570,000 from landowners. Soldiers built and maintained a lot of the roads in the empire's periphery, but in regions like Italy the work was often done by mancipes who had work teams of contract workers, captives, and slaves. While the Han road network also required maintenance, this was done by corvée labourers conscripted by the ministers. In early winter each year, these conscript labourers would be required to clean, re-tamp, and re-face roads under ministerial direction, and emergency repairs could also be ordered whenever large holes formed or passage became impossible.

It's also worth noting that travel along Roman roads was generally freer than it was in medieval Europe, classical China or medieval Japan, where tolls were abundant (they were a rarity in the Roman Empire) and the kinds of vehicles that could be used were restricted. There were restrictions on the movements of slaves, exiles, and senators outside Italy after the Republican era, but nothing of the scope of the restrictions on female travel in early modern Japan or the carefully stipulated limits on what kinds of people could travel and when and what paperwork they would require in Han dynasty China. Travellers in the Achaemenid empire left their place of origin with a sealed authorisation from the king or satrap (provincial governor) which included the details of their group's composition, the journey they would take, and how much they should receive from the wayhouses in rations. Presumably it was also required to stay in the wayhouses.

Nothing like the Roman road system existed in medieval or early modern Europe. Roman roads often filled a similar role to roads built by earlier Near Eastern empires, though they were more impressive in their durability. There is a strong competitor to the scope of the Roman system in the road networks of China, but there were important differences in terms of how they were built and maintained. In my opinion, it's a mix of their scale, the quality of their engineering, and the social and economic differences between their operation and the operation of road networks in other empires that makes them so significant.