Why do Zerg players build their Evolution Chambers quickly?

I often see Zerg players build spawning pool immediately followed by an Evolution Chamber (EC). What is a purpose of that quick EC? To get quick armor/attack upgrades for zerglings?

Sometimes I also see a Zerg build EC without building extractor... so how he is going to get gas in this case?

Solution 1:

I'd say it is a quick and cheap way to free up some supply, for a building you'll need eventually anyway. Might also be used for wall-ins/sim city.

Solution 2:

There are two reasons for the quick Evolution Chamber.

Reason One

The first, as you noted, was early upgrades which can quickly turn a fragile force (take zerglings for example) into an early game force to fear. However without gas this isn't a feasible strategy. If attempting this build order, gas first is a must to make use of that early Evolution Chamber.

Reason Two

What you are witnessing is preventative measures designed to protect against early air pressure. With Banshees, Void rays, and Mutalisk a consistent threat, a Zerg can unlock the ability to build spore crawlers providing both detection and vital air defense. The extra protection is often worth the extra resource cost to help the Zerg survive to mid-late game where they can overwhelm their opponent with macro.