Zerg rely on the high-energy state of Vespene gas to power their incredible metabolisms and fuel the accelerated growth of their larvae. The Extractor is essentially a giant organ that sits atop a Vespene geyser and packages the gas into small flesh-wrapped sacs that can be easily carried by Zerg Drones to a nearby Hatchery for absorption. Since the Extractor lives on Vespene gas itself, it does not need to be built on an area already covered in Creep.