Let's say that a GM decides to run her favorite setting: Dark Sun. She runs player driven improv based games with an eyes always toward the PCs being the protagonists. One player says "I want to play a Warforged," and the GM says , "No, I want to keep this one by the book. Pick a Dark Sun race, please." Suddenly the GM is one of the worst thing for the hobby?
You don't see how utterly ridiculous that sounds?
I'm currently running a game where the characters were teleported to (my version of*) Dark Sun.
One of the characters is a Warforged, so I've added them in, with a simple explination that they're said to come from a distant land to the north. NPC's might refer to the player as 'A child of the ore' or some such terminology. So this is a basic example of world building as a plot hook rather than a restriction. I've found they fit in really well from a mad max vibe, which Dark Suns does pretty well.
*I've been really inspired by the recent Spelljammer set to do a Dark Sun/Jammer mash up setting, with the idea that there was an age of falling stars when a bunch of Spelljammer ships crashed on Athas. The premise makes it very easy to add anything to the setting and lets me have fun world building to suit my players.