Wolff always sends the striker spring or firing pin spring with an order, the difference is explained below... If you experience light strikes to the primer without ignition you may need a new striker spring. 
If you have a hammer fired pistol and experience drag or pin wipe on the primers during firing a new spring may help.
Hey look at this way, even if you don't use it, you have a replacement in hand.

STRIKER SPRINGS
Fits All Glock Models
W. C. Wolff Company offers reduced power, factory standard, and extra power striker springs (firing pin springs) for Glock pistols. This spring will help improve the trigger pull. 
Warning: This spring is for competition use only - not for duty use.
Warning: Do not use extra power striker spring with reduced power recoil springs.
Reduced Power....: 4, 4.5 and 5 Lb.
Factory Standard..: 5.5 Lb.
Extra Power.........: 6 Lb.

In their FAQ's...http://www.gunsprings.com/faq

9. What is the difference between a firing pin spring and a striker spring?
A firing pin spring is actually a return spring as it returns and keeps the firing pin retracted.  The firing pin spring works in front of the firing pin pushing the firing pin away from the primer usually keeping it retracted in the slide.  When the  firing pin is struck by the hammer the impact force of the hammer overcomes the retraction force of firing pin spring and drives the firing pin into the primer.
A striker spring is actually the spring that causes the firing pin to striker the primer.  The striker spring works behind the firing pin.  When the gun is in the cocked position, the striker spring is compressed behind the firing pin. When the trigger is pulled the firing pin is released and the striker spring pushes the firing pin into the primer.  While technically incorrect, a striker spring is often referred to as a firing pin spring.

Shadow thanks for the clarification. The firing pin springs have been confusing to me.

Your welcome, its all about learning and knowing how things works, the ultimate goal is Safety! 

Shadow, I'm not complaining that they are including it, I actually appreciate it, and I understand it's purpose, I'm just wondering why they do it. It costs them(me too) more to include it and I don't think anybody would complain if they didn't...there must be a reason they go out of their way to do this?

2 range trips with the Gen 3 today...one was a massive failure the other was great.

Trip 1: fired 4 magazines of DT, each had at least 2 FTFs...some were so bad that the round was vertical in the magazine.

I suspected that the Wolff recoil spring was the problem because it wasn't as hard to rack the slide today (only fired 159 rounds...wtf?) I returned the gun to stock recoil spring and stock barrel for range trip 2.

Trip 2: 6 magazines of DT...no problems.

I will be carrying this gun in stock configuration. I will also make another range trip tomorrow...

And I just bought some backup SP05586 recoil springs off of Midway...

Went to the range this weekend with a buddy.  G20 g3, kkm, and updated to 22 lb stainless steel rod with new slide stop lever.  No FTF from my use or my buddy.  We ran Remington 180, Underwood 165 TMJ and BB 180 JHP.  No issues and the slide stop with 22 lb seemed to cure my issue.  Went to Glockmiester and the guy said hmm yeah go to 22lb and replace the slide stop lever.

have fun!

Shot another 112 rounds stock out of the SF...all good.

www.glockstore.com has dedicated gen 4 dual recoil springs, however for higher power ones they are under the competition ones but only offered in the little Glocks 29/30/36

Update: I've gone to the range 3 more times since my last post. All 3 times with the Gen 3 SF in stock configuration....no problems with Underwood 180 Gold Dots and Double Tap 200 grain Noslers and Hardcasts.

So stock G3, no issues.  Congrats!

Guys,

Was following this thread and I have a new Glock 20 Gen 4 I purchased a while back. I took it to the range this past week and fired about 250 rounds through it. I have a 6" KKM barrel and added the extended slide release and upgraded the sights. I bought a new 22lb spring from Glockmeister recently but when I went to the range I left the stock spring in. I shot about (50) PPU 10mm rounds (weak ass rounds) and then shot (200) 10mm loads I had worked up myself.

I have to say I didn't have a single FTF or any issues at all. I'm running the stock 15 round Glock mags since I've seen problems with the 10 round mags feeding for many people. But again, so far had no problems feeding. My reloads were using Hornady XTP 180 grain hollow points and also some were loaded with 180 grain Xtreme Flat nose copper plated bullets. Next trip out I'm going to swap out the factory spring for the 22lb Glockmeister spring and see how that does.

One thing to note is I polish all my feed ramps on my automatic pistols to help the feeding process and to reduce hang ups. Since I was working up some 10mm loads, my charges went from light to very stout and no problems with any feeding. 

Midwest, that's good to know. I too have a G20 gen4 with approx 400 rounds now through it. It has always cycled the weaker stuff & Double Tap 200 WFNGC fine, but early on I had many FTF's with Underwood 180 & 200 tmj's. At first, a lot of problems, particularly with the 180's. This seemed to be consistent with other posts here. I started to try heavier RSA's but decided to shoot it stock for a little longer. First the FTF with the 200's declined, then 180's. Last time out I shot a box each of UW 180 & 200 tmj's with no problems at all. I know - not the largest sample size, but it definitely seems things are improving greatly as the gun gets broken in. I sure hope this trend continues as I absolutely love the 10mm round and the G20 platform.

Just my experiences - interested if others who have had problems with Gen4s have seen a decline in those issues as round count increased.

Yah, knock on wood, my G20 Gen4 is working the way it's suppose too. The Glocks are pretty loose tolerances compared to most steel frame guns and as mentioned earlier in this post the lower frame flexing could be some of the issue. I noticed on my Gen4 that with the gun assembled, the lower receiver is actually bending or flexing from the pressure of the spring. This has to make me wonder about the alinement of the rails during cycling and if their might be some binding going on especially under heavy loads.  I also think there's an issue with the 10 round mags feeding properly in the G20's. But so far mine's working and I'm hoping it stays that away. It will be interesting to see if the 22lb spring works as well or creates any issues. I'll find out the next trip to the range for sure.