IG army compositions

[Tergt69]

Hey guys i started playing Elite 1 year ago but then dropped it for some reason (i cant really remember why) and picked it up again recently.
I was looking through the forum to get myself back up to date and in one post within the strategy guides called "Imperial Guard vs Chaos double CSM and Heretics opening." and i saw a very interesting thing there:

"If you are an IG: Opening strategies include:
Lord General: 2 GMs, no sent, 1 cata/hwt/arti spotter and try rush to t2.
Inquisitor: 2 Gms, 1 sent, 1 cata/hwt/arti spotter; and try hold ground until t2.
Lord Comm: 3 Gms, maybe a sent, and maybe 1 cata/ hwt/arti, and try somehow win."

(this is quoted from the original post of the discussion)

What i was thinking about what makes these combinations work beter with the specific commanders? I usually run the following with LG and LC (i dont really play INQ) in 3v3: starting GM + Sent +GM + HWT +1 arti/catachan if needed but this last purchase is situational. I tried what was writen in the quoted post for LC and i have no single clue why, but it worked very well (3GM+4th unit situational but still a "must" buy +5th depending on heavy T1 investement or not)!

Could someone enlighten me why these combinations fit these commanders? It would be awesome to know why i am doing what instead of copycating others strategies without thinking about them

[egewithin]

You shouldn't make a spesific opening every time. Your unit desicions totally depends on matches it self. Let me give you some examples ;

Do I get Sentinal? Oftenly. Generally a no branier against SM and Eldar, useful against nids and Orks. It gives much better dps and range then normal Guardsmen against SM and you can use it as a wall. Tanking incoming damage and out repairing it. If you can't, fall back. That is it! It also give protection for my GM in case of melee threats like Banshees and ASM. Sentinal is also a very useful AV option in T2. Can run after Razorbacks and Falcons, extra fire powaa against Dread and Wraithlord.

Why I do not get Sentinal? Because enemy can out shoot it and can not survive untill T2, and goes down fast in T2 by some high damage output races A.K.A Chaos Space Marines. Also, repairing it decreases my micro and takes a lot of time sometimes.

I oftenly go Catachans since they are anti-everything, but not so wise against all ranged armys like 3 Shootaz or 3 DR. In that case, HWT or Spotters are much better. When the enemy shows his cards, you play your counters against it.

[Atlas]

Skipping Sentinel vs Chaos is a really bad idea, and it's probably the matchup where it has the most impact in t1. All 3 Chaos heroes has some way of forcing through your HWT and you do not want to let a Chaos melee ball rolling.

[Impregnable]

Since it might be too long to post here, I sent you a private message regarding this topic.