This building is mutated for converting the Vespense gas for the usage of the Swarm.This building’s cost is 50 but the drone is 50 mineral too so the total cost is 100 mineral.At very early stages of multi matches, mutating Extractor can be used for different purposes, you can mutate it at enemy vespene geyser to observe him or you can mutate an Extractor at your geyser than you can make an extra drone after that you can sell the Extractor.This process gives you extra drone without morphing Overlord.