D&D is sometimes distilled to a formula of fight monster, take treasure. It's oversimplified, certainly. There's much more to the game than that (or at least there ought to be!) but you'd be hard-pressed to find a D&D campaign in which that trope is completely absent. It raises an interesting question: Just what are all those creatures doing walking around with pockets full of gold and silver coins, or holed up with piles of them? Where do they get them and what do they do with them?
Some unintelligent monsters might be like magpies, hoarding "shinies" both valuable and worthless; others might just accumulate odd bits of incidental loot carried by their victims, left forgotten in the piles of bones. For dragons, it's an obsession, pure and simple, with no practical purpose.
But what about the evil humanoids - orcs, goblins, bugbears, ogres, and their ilk? They're not mindless hoarders, nor incidental collectors. I've never seen them portrayed as obsessive or compulsive about treasure, as dragons often are. They're not great appreciators of the beauty of precious metals like dwarves and gnomes. They can't eat gold and silver, nor make anything of practical use out of them. No, humanity's brutish fantasy cousins value coins for the exact same reason that humans do: they can be exchanged for stuff.
The most common assumption, I think, is that orcs and goblins live by plunder. In the most simplistic model, they raid for coins, gems, and jewelry. In the slightly more realistic one, they take food, cloth, metal goods, ale and wine, slaves, livestock, and whatever else they can lay their filthy hands on. The truth, though, is that orc does not live by raid alone.
Raiding is a pretty dodgy business. One week you might get a herd of sheep, the next a load of iron pots or a shipment of linen or a dozen captured caravan guards. If the village nearest your lair is known for growing potatoes, that's probably what you'll get most of the time, even if you really need a good stiff drink or some new pants because your old ones are so full of holes they barely cover your orc-junk any more. What's a humanoid to do? Trade! Exchange what you have for what you want. Barter works, but coin opens up a lot more possibilities.
While settled humans and demihumans are a lot more industrious, and have a much greater variety of useful stuff to trade, they're not exactly open to it. Respectable people don't do business with monsters. An orc can't just walk into the local tavern and order a shot, nor visit the village mercantile for a bag of wheat and some new hobnailed boots. It turns out, though, that human and demihuman bandits and brigands make excellent middlemen. Other than being a bit rough and grubby, there's little about the average highwayman to arouse a merchant's suspicions, at least not enough to make him turn away perfectly good coin, and the highwayman has few scruples about trafficking with the orcs who raided the very same village the week before. Such people might well be willing to sell their excess food, clothing, and weapons to the orcs in exchange for coin, or buy the wheat the orcs have stolen and cart it to the next village, pose as farmers or merchants, and sell it. It's not only possible, but likely, that the local orc and goblin tribes have ties with the local criminal enterprises. If there's an area of your campaign world where slavery is an accepted practice, they almost certainly do a brisk business with human slavers too - even (maybe especially) if the slave-holding nation is some distance away.
Trade between the various tribes would be common and frequent, too. The hobgoblins that live near the dwarf stronghold might jealously bar the lowland orcs from raiding there, but they'll gladly trade dwarven steel for purloined mutton. Goblins who find themselves in possession of a surplus of tanned hides might negotiate an exchange with the forest kobolds who have a ton of squirrel meat. If the kobolds don't need hides, they'll probably take coins, which they can turn around and use to buy spears from the gnolls.
Such trade arrangements might be stable long-term arrangements, but could just as easily be quite tenuous and volatile. If a band of brigands moves into the area and offers the bugbears a better deal on stolen food than the orcs, the bugbears might turn on the orcs, and the orcs might launch a turf war against the brigands to protect their niche. Weaker tribes might well be the biggest wheeler-dealers of the local monster populations - if they can make themselves useful or even indispensable, they have less to worry about from the stronger tribes, and might even gain their protection. Those kobolds might look like easy pickings, but the bugbears don't want to have to hunt squirrels for themselves, so don't mess! In the human world, describing competition as cutthroat is just a metaphor, but among the humanoids, it can be a deadly reality.
If you spend a little effort outlining the trade and diplomatic relationships of the various humanoid clans and tribes and their shady human partners, you won't be caught guessing about the repercussions when your players wipe out that cave full of orcs. You'll have a good idea who's going to be angry and seek revenge, who's going to be in dire straits if they don't find a new source of weapons, and who'd like to shake the PCs' hands for eliminating a rival. All it takes is a little applied orconomics.