1. The opponent has to spend a ludicrous amount of minerals on static defense.
2. You can start upgrading air carapace pretty early.
3. You can expand a lot easier than him.
4. You can pick off drones if he doesn't have enough spores or hydras.
5. You can pick off hydras with micro until he masses far too many, in which case you should have been able to amass a large number of upgraded mutas and lings.
6. He can't really harass your expansions, and kinda has to go for a one-hit kill attack.