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.
gaming.stackexchange.com/users/6431/Aardvark
Yes, to increase the effectiveness of early Zergling/Speedling harassments, early upgrades are good.
In the case that you see an Evolution Chamber without an Extractor, he unlocks the Spore Crawler .
gaming.stackexchange.com/users/998/Tamara Wijsman
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.
gaming.stackexchange.com/users/4378/Per Alexandersson
I don't agree with any of answers doesn't handle case of EC necessity in EARLY game (is build together with Spawning Pool w/o gas).
Gas is absent, so you won't be able to start upgrades; Opponent Air Units will be absent yet 2 or more minutes, so there are no value from ability to build spore colonies
My guess, why they build EC very early is they are just supply blocked and want to release 1 supply.
It is not too reasonable though, as they can build extractor (it is only 25 minerals)...
gaming.stackexchange.com/users/3467/Budda
It can be effective.
It DOES make a person who scouts you not think about putting out early air.
Its a decent fooling tactic.
Additionally, if you are capped out, have no larvae, and have huge mineral surplus.
You need that building later on, but will you have the time/money/drone when you have to produce a consistent stream of army units?
Do you neglect to build a roach warren if the other person is getting something that can counter roaches?
Even if you tech up, maybe they build something later that completely destroys you because you didn't have a roach warren built and couldn't produce them in time.
And because you didn't build the building before you needed it, but only after.
gaming.stackexchange.com/users/None/
Getting an evolution chamber early is usually a mistake.
Unless the zerg player took two extractors early, they won't be able to make the upgrades, and air/cloaked units will take a while longer.
It's better to put those minerals into getting an expo up.
I usually don't get an evolution chamber until I have my expo ~50% saturated, and by then I can get an overseer scout to see if there's any air production.
The exception is if you're able to scout a VR, banshee, or DT rush, because those can occasionally come before lair tech.
Personally I have experimented with a build that gets early +1 speedlings to defend the expo and deny scouting and fast expands.
But of course that requires gas.
gaming.stackexchange.com/users/11378/Kirro
Maybe he had a defensive tactic in mind.
When evolution chambers die, they spawn broodlings.
Broodlings aren't as a secure a defense as spinecrawlers are, but they may be a decisive factor when defending a base since they attack, and they may give the player some time to return to his base to defend.
Tt gives sporecrawlers and upgrades, if the Zerg needs some.
gaming.stackexchange.com/users/13081/Guilford
Tonight I learned that carapace before speed is really really really good vs marine pressure openings.... slow lings with +1 armor do way better vs marines then speedlings without any armor.
They actually do better vs marines then +1 attack speedlings, if the marines don't try to run away etc....
The issue is there speed and the difficulty in getting surrounds with out speedlings....
but your going to get speed shortly after anyways.
Here's another weird tip that sounds like it can't work
but it does.
+1 carapace with lings is better then +1 attack vs unupgraded stalkers as well.
BUT +1 attack is way way better vs stalkers if they get the armour upgrade.
I'll leave it to you as an excercise to figure out why.
I would go as far as to say that evo chamber at the same time as your pool hard counters marine + tank openings and pure bio openings.
(NON-reaper)
gaming.stackexchange.com/users/47658/furyOfSkanks
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?
Why do Zerg players build their Evolution Chambers quickly?
gaming.stackexchange.com
<starcraft-2><starcraft-zerg>
gaming.stackexchange.com/questions/13997
gaming.stackexchange.com/users/3467/Budda
Early upgrades are good, it increases the effectiveness of Zergling/Speedling harassments. It is a quick and cheap way to free up some supply. It is generally agreed that it increases effectiveness of early harassments, and is a cheap and quick way to free up some supply. It also be some kind of defensive tactic. Some do say however that doing so is a mistake.
Users suggest that doing so has some advantages. However one user suggests that it may be a mistake on the opponent's part.