If you've wasted any amount of time reading about PC gaming, you've probably read at least something about this or that game having old code and content, not active, but left in the game. Half of it doesn't work and never will because it's incomplete. But sometimes there are useful bits left in there. Modders love this stuff, because it's a jump-start on being able to add new content. Re-enable a disabled enemy type, give it some spawn points, fix the hit box, and bam, you've got more content. I've not yet tried it, but KOTOR2 had a lot of this stuff left over, cut by a rushed schedule, leaving modders with a lot of material to work with. How glad we should be that they didn't prune it!
WoW had this as well. As expansions accumulated, so did the random stuff with no apparent purpose. Quest chains that seemed to end too soon. Ominous bosses. Factions which increased, but for no apparent benefit. Nooks and crannies in instances. Why was it there? For some grand plan? Were they setting it up for a Soviet-style five year plan to use it in five years and then send it to a work camp? I doubt it, and not just because of the image of a Blue sending a cut troll boss to Siberia.
Instead, I think they were just being thorough and sloppy. Why did Shendrelar have a reputation? Maybe it adds some sense of consistency, of "thanks for bringing us books, bring a hundred more for a discount on things you'll never buy*". It was consistent in the way that slapping a reputation on everything is consistent. And utterly nonsensical. The result was a faction that existed for basically no point at all.
Ravenholdt Manor was similar. Rogues needed some place for a class quest, so they got it. And then for no apparent reason, the guards there were a faction. Maybe it's a coding thing, that guards need a faction to properly kill you for misclicking on a PvP server. Or it's just insanity, a theory which is supported by the existence of not one, but two grinds for increasing reputation, both based on rogues, and none of them giving any material benefit. Though the guards are nicer.
WoW ended up with a lot of shit laying around.
If they were truly polishing everything, they'd get rid of it. What is it doing? Taking up space? Wasting space in the intertubes for one more mob, one more zone, one more pixel of pointless existence. These are all shit left around by developers who clearly could not be bothered to go back and clean up the mess they left.
And good thing they didn't!
Going back to Insane in the Membrane, what was that achievement? It was the cobbled-together pack of utterly pointless reputations along with one essential one for the well-dressed pirate. Can you imagine trying to create that these days? There just aren't enough useless, left-over, backwater nothing factions to work with. There are no bits of scrap laying around for the found art project which is this achievement.
But that's not all!
Reduce (didn't happen), reuse (coming to that), recycle (lol, every boss in MC).
Blizzard was a strong practitioner of the Reuse model. They created the Dungeon 2 set, which all the cool people called tier .5, since of course the dungeon set was tier 0. If you're unsure what I'm talking about, imagine if the set pieces that you normally purchases from vendors in Stormwind (which is so annoying for Horde characters needing to run all the way over) were instead random drops, and they came from bosses in instances, scattered all over the place, so that running the same place a million times wouldn't get you a full set. Instead you'd need to run a dozen places a million times, because some jackass ninjaed your set piece. I also once got an epic staff on my warlock, which the nice priest passed on, thanks to my cunning plan of not ninjaing his drop earlier in the run, and then I got called a ninja for having it. Apparently I should have forced the priest to take it. But I have once again rambled on about something entirely unrelated. Maybe after a few dozen years and a touch of dementia this will be understandable behavior.
Okay, tier .5, what would you do if you wanted to add a bunch of new content and a quest chain that tells a neat story and upgrades a set of gear? Okay yes, you could tell people to run heroic ICC and trade in their tier pieces, but that doesn't have a different story and dear god, I cannot believe anyone willingly ran that place again after killing Arthas Mc Jackass the first time.
Or, you could go with the slightly less lazy route and tell a story we've never heard before. In this story a set of adventurers went off and got something awesome, then promptly found themselves cursed by how awesome it was, before finally it destroyed everything they had ever held dear to them. I think it was supposed to teach us a lesson about legendary items and guilds. These adventurers met terrible fates in the very same instances you'd farmed mere hours before. So of course you have to go back and kill them, because that's how we solve problems. Anyway, they went back through the instances and added in strange new boss fights with strange new gear. Imagine if mages and warlocks got a special set, a set whose special qualities included extra armor and basically nothing else. And it was awesome. But that's exactly what I'm trying to get at: when they needed more content, they already had all this places, some unused, some slightly used, like stuff from your garage that you'd sell on ebay, and they stitched it all together into a very nice quilt.
I'm not adequately explaining what a nice quilt this way. Across the story they manage to draw in other stories, everything from demons to undead to angry dwarves to not-entirely-mentally-stable elves. Remember those utterly pointless Shendrelar? Well there you go: They're being used again (technically the Insane used them second, but I like to tell my stories out of order). It's like a web of conspiracy we'd ask Glenn Beck to create, showing how the Argent Dawn, Orgrimmar, Silithus, Dark Iron dwarves, demons, and a whole pile of gold out of your pockets are all connected. DON'T YOU SEE? AM I JUST CRAZY OR IS IT JUST ME!?
Unfortunately, Burning Crusade failed to create quite so much random crap to work with, leaving the developers with no choice but to create new content. It was a terrible shame. Wrath of the Lich King did even worse, creating a nice clean continent with only a few stray dragon portals to tie in to. Then the Cataclysm came and all the useless crap was swept away, never to be seen again, and that made me a bit sad.
A highly polished experience is a good thing to have in a game. But a highly polished world, not so much. If we wander off the tracks and go poking around, we should find something dirty. Think of the stories. In one story the world is a perfect mess with grime everywhere. In another, the world appear perfectly clean, but if you go behind the wrong door you will find the horrifying mess. But in the most terrifying world, everything is clean everywhere, and that's how you know it's a nightmare which you cannot ever escape.