In World of Warcraft, there are raid instances that reset once a week. The instances consist of several encounters that usually have to be defeated in order. For example, Gruul’s Lair, the easiest 25 man raid instance, has encounters with High King Maulgar and then Gruul. The hardest instance, the Battle for Mount Hyjal, includes five encounters.
Playing all of these encounters takes time. The fights themselves are time consuming, and Blizzard includes “trash” monsters between each of them to increase the overall time it takes to complete the instance. In each instance, the encounters get progressively more difficult. So teams that can kill the one boss cannot necessarily defeat the one that follows. In game terms, this is what progression is all about. Every time you win an encounter, you get more stuff to enable you to move on to the next boss.
What winds up happening, though, is that teams (usually guilds) progress as far as they can and the instances reset at the end of the week, leaving some hard bosses alive. This creates a market opportunity for those guilds in that they can sell their unfinished instances to guilds further along in their progression. The sellers can’t kill those bosses anyway, and the buyers get the opportunity to spend their time on their own progression rather than killing bosses that don’t drop anything they really need.
I find it really interesting to see economics provide a solution to a common in-game problem, which is that doing the same raids over and over is time consuming and to be honest, incredibly boring. Progression-oriented guilds need the loot that the later bosses in raid instances that they’ve already conquered drop to complete high end armor sets and upgrade their weapons, but to continue their progression they need to avoid spending entire evenings rehashing content they’ve mastered. The market delivers.