It speeds up your economy far more than you'd expect. Not only do you get two Drones instead of one, but because those Drones will be mining minerals it allows you to buy them - and your macro Hatchery faster. More importantly, because Drones are now only worth 1/2 supply, that means you can get a lot more mining before you need to spawn Overlords to increase your supply, and it means you have more supply available for your army.