Importance of Zerglings to Zerg armies

So there are a lot of factors here and I'll try to break down them into categories:

1. Why do I get Zerglings
2. When are Zerglings good
3. How do you use Zerglings

The short answer to your question is: Zerglings are not necessary (by any means), but they are a very useful unit in a large number of situations.

Why do I get Zerglings?

The question of why people get Zerglings is an important one so let's first talk about what Zerglings do well.

Zerglings are the most cost effective damage per minerals in the entire game. There is no unit which (for cost) is as good at tearing things down. Looking over my charts, Zerglings do a whopping 28.74dps for every 100 minerals (unupgraded) the next closest unit is stimmed marines at 20.9 (for reference a Zealot does 13, a charged VoidRay 6.25 vs armored and a Hydra 9.6). So when it comes to laying on damage there is no better choice. If we want to look at hit points, we see 35 for 25 minerals (a Roach is 145 for 100 minerals) so there is no doubt that this is a very cost effective unit.

Going beyond how cheap they are, Speedlings are incredible fast units (4.7 off creep, compared to upgraded Reapers 3.8 or Hellions 4.25) which give you a large amount of map control. This combined with their small size and high dps means they are very effective for harassment and hit and run tactics.

So when we look at why people get Zerglings it is for two major reasons:

1. Cheap unit
2. Great harassment
3. Banelings

both of which the Zergling excels at (I know I put a third one there, but this is about Zerglings; Banelings are covered elsewhere).

When are Zerglings good?

Obviously they're a very effective unit early game, but this is largely a result of the problem: teching takes time. Instead let's look at when Zerglings aren't good.

Zerglings aren't good when your opponent is getting units which are effective against them. Reapers, Hellions, Zealots, etc, you probably know the drill by now. Because people often open with these units it definitely leads to perception that Zerglings are an underpowered unit. What's more any ranged unit that works in large groups (eg. Marine balls) really reduce Zerglings effectiveness.

Because Zerglings are a melee unit the "surface area" of your opponent's army is an important factor in how to use them. Shapes with a large surface area (eg. a line) allow for many Zerglings to attack at once. Shapes with a small surface area (eg. a circle) allow for fewer Zerglings to attack at once. As a result, even though you may have many more Zerglings than he has Zealots (for example) if he clusters his Zealots together they can over come these numbers (think the Spartans in the movie 300). Terrain also plays a role here. If his units span a gap, then you have even less surface area your Zerglings can access.

Finally, Zerglings really suck in small numbers. This is the biggest mis-usage of Zerglings I see. Someone builds 24 Zerglings and sends them to fight 10 Zealots. I know 24 Zerglings feels like a lot, but its really not. Think of supply for a second. 24 Zerglings are 12 supply, while 10 Zealots are 20 supply. While supply isn't the greatest indicator of a units effectiveness you can quickly see why you really don't have enough Zerglings for this fight. If I'm going Muta/Ling and I want half my force to be Mutalisks, half to be Zerglings, and to have about 60 drones, that means I should have about 140 Zerglings. If I have 40 or even 60, I'm just not building enough.

So Zerglings are good when

1. You have a lot
2. You can pick where you fight
3. You're not against units which really hurt them

How do you use Zerglings?

There are really two parts to this question: the Micro and the Timing.

Zerglings require a decent amount of micro. Despite what you've seen with AI pathing and auto surround, the AI is actually pretty dumb. If you stick your Zerglings behind Hydralisks who are attacking, they'll get stuck back there. Additionally, Zerglings will target whatever is closest to them (frequently Zealots) instead of what they excel against. Finally, a Zergling will run right up to a unit and start hitting it. While this works well for units standing in place, any unit on the move means the Zergling will get in one hit before it falls behind. It also means that its brothers now have to run around it to hit the target.

Reference Material

Notice how the Zerglings just run up to the Reaper and only get a hit or two in before it gets away? Now imagine, if you will, if instead of attacking the Reaper the Zerg player had moved (not attackmove) behind the Reaper. As the Zerglings spread out in a line (as they do when moving) they would be in a line passing the Reaper. Now when the Zerg player attacked, the Zerglings would surround the Reaper, preventing escape, and do more damage, even if it did escape).

While this Micro seems relatively trivial it is essential to proper Zergling usage. It not hard to practice either. Get used to moving your Zerglings (not attackmove) past your target and as they pass by, then attack.

Timing is another tricky thing with Zerglings. Because they require so much surface area attacking a large MMM ball (or an entrenched position) is rarely a good idea. Instead, find opportune moments to run your Zerglings into his mineral line, or kill of proxy pylons, or attack his MMM ball when its spread out. You'll find your Zerglings are far more effective and do a lot more damage.

Take Home material go to 34m 19s

So should you learn and start using Zerglings? Without question. Having another unit in your arsenal is always a good idea. However, Zerglings have some intrinsic properties which make them extremely useful and your play will benefit from them.

This is a supplementary answer, I think tzenes' covers most of the important topics.

I just want to emphasize the importance of upgrades to lings effectiveness.

• If you are going to use lings, I believe you should get the speed upgrade for them ASAP.

• If you are still using lings when you get to hive tech, don't forget the adrenal glands upgrade. For less than the cost of an ultra, you can make your already high dps lings 20% better at razing bases.

• A Ling heavy army benefits greatly from attack and armor upgrades. +1 attack may not seem like much, but it is a 20% increase in damage. +1 armor may seem like not much, but as tzenes points out, it can mean the difference between needing 3 shots to kill vs 2 shots by zealots. That's a 50% improvement!

Starcraft 2 has auto surround, so micro won't be that much of a problem.

The reason why many zerg players depend heavily on zerlings is because many zerg armies are very gas heavy. Mass zerglings will soak up the excess minerals without taking away from your gas units, like roaches would.

Also zerglings are better than roaches against many units, like the stalker. Advise against hydras for anti ground in general...

This is a supplementary answer too: The main reason of Protoss and Terran wall-in the base with pylons/supply depots and barracks/forges: Zerglings. And in the late game, sometimes they dismiss the wall, a great oportunity for cracklings :)