Frequently, Dungeons and Dragons takes PCs well beyond the bounds of civilized lands. The most common settings are those at the very marches of settled kingdoms, facing wilderness, foreign enemies, humanoid foes, and other such nastiness. Commonly, the great empires of the ages past are little more than memories, leaving their marks in the form of ruinous welts upon the land. Adventurers are warned to pack a lot to drink because streams and rivers may not be common, to load up on food because the hunting may not be good, to take everything they might need with them and be prepared to walk miles in the wilderness away from all civilization and without good roads, trails, or markers.
So what happens when your adventures are no longer taking place in a great and wild wilderness with only a single large city nearby to call home base? What happens when, for example, you adventure in the capital of all civilized lands, in the heart of human settlement, in the Mother City of Miles? Well, you can reap the rewards of settlement and cultivation, that's what.
The first, most vital, and overlooked thing is that water is plentiful. Great cities are provided with public wells and fountains. The roadsides (and yes, these are real paved roads, not just the simple dirt tracks most kingdoms dare to call highways) are dotted with convenient locales to refill your water, allow your mount to freshen and drink or just graze. Water, the stuff of life, is critically important to adventurers whether they realize it or no. You can't just travel assuming you're going to get the chance to fill up your water skins every single day in the wild—unless the "wild" is the long snaking highway of the Pillar Road, or the Coast Road, or the Oakway, or the Greenway. Yes, there's something to be said for civilization after all.
The second important element of a well-maintained public works is that of the waystation or mansio or viator's stop. These can vary from a small inn-house to a walled town, but in all cases they exist solely on the emperor's coin and primarily in order to provide imperial messengers (viators and Sacred Heralds) with fresh mounts each day. In order to defray the cost of operating mansios and waystations, the emperor's servants often take in guests and caravaneers traveling the highways. Sometimes they forward the excess fees to the emperor and the imperial purse actually makes a profit, but mostly they just keep whatever's left over for themselves. And why not? The notion of corruption and misappropriated funds is much weaker in Arunia as people aren't expected to separate public and private personae.
Civilization also provides good access to healers. Wherever a city stands, physicians and medici must be in attendance. These men and woman can only be supported by major urban centers. In all other cases, a town or ville is likely to have only one or two, a flock of less reputable folk, and perhaps a few cunning men or women who make their living by mumbling magic words and giving folks nettle tea.
Demographic diversity is also much more prevalent in large urban centers like Miles. These cities rely on their position on trade routes (or trade routes rely on the wealth of these cities, depending on the details) and trade routes bring contact with the outside world. Dwarven forges, elvish workshops, and gnomish cartwrights can be found in profusion in Miles—and where there's profusion, there's price competition, tending to drive prices downward (though the expense of living in Miles drives them back up again; the battle of pricing is a difficult one in the center of the empire).
There's always plenty to do in great urban centers for adventurers, from exploring the ancient and august sewage system inherited from the Second Empire to being the political bullies of one faction or another. Factors, magnates, merchants, and more all have needs to be seen. While there may be a dearth of dungeons proper, the other types of work more than make up for their presence (or absence). Smuggling rings, assassinations, tavern brawls, and more will provide myriad and sundry options for those thrill-seekers who choose to stay in Miles or any other great city.