Before I embark upon an Imperial Guard review an Armies in 5th series is necessary! Imperial Guard are obviously a very shooting focused army with most of their Force Organisation slots having a lot of ability to get effective ranged firepower. This isn't what Imperial Guard are all about however and they can make very aggressive armies based around AV12 and the all important Chimera. Even backfield shooting armies are likely to have several of these transports in their list for added firepower, protection and mobility which is important to be able to grab and contest objectives whilst the range firepower pummels the opponent. Let's look at this more closely.
For the most part, all of the tanks available to Imperial Guard have AV12 or higher on their front armor and due to their cheapness (55 points for a Chimera), you can easily field 10+ tanks in a given army. Add in the squadrons in heavy support and fast attack and that can easily reach 20 which is an insane amount of armor to deal with. The downside of this of course is their weak side armor. Whilst many of the tanks (Vendettas/Valks, Russes, etc.) have similar side armor to their front, the Chimera which is what often adds mobility and durability to an Imperial Guard armor has side armor 10. This compensates for their front AV12, two heavy weapons and ability to fire five weapons out of the top hatch but doesn't detract from an Imperial player's ability to make a literal wall of AV12 in midfield for relatively cheap.
This helps increase an Imperial army's durability as their basic guy is T3/5+ and therefore pretty easy to kill. A bog standard explosion for example is going to take 3-5 Guardsmen/Veterans with it and that's not a great ratio. Having a ton of tanks in midfield which the infantry inside can fire from however, well you're presenting a double threat to your opponent. First you have the ability of the tank to protect the infantry inside whilst also having two heavy weapons itself and allowing the infantry to fire out of the top hatch. You then have the infantry inside themselves so even if you suppress the Chimeras, they can hop out and slag any nearby units if it is necessary. This is all heightened by the ability for multiple Chimeras to cover their side armor and present a very large AV12 frontage. Even with an army not based around this concept but rather ranged firepower, there could still be five to six of these tanks in midfield holding infantry units which are very capable of making life miserable for tanks and gives the Imperial army a hold on midfield.
This hold isn't as strong as many Marine armies because they lack the durability of T4/3+ models but the increases number of models and more durable tanks in the front arc gives the Imperial army a significant advantage here. If they are able to maintain several Chimeras in midfield without getting flanked (i.e. side shots), the Imperial army can use that wall of AV12 to anchor the rest of their moves and present a much more significant threat bubble to their opponent. If that AV12 wall disappears (out flanked or simply powered through) or there isn't an AV12 wall to begin with, opponent's have a lot more options in how they approach an Imperial army. Whilst the firepower they face downfield is more than likely more significant, there is less need for an opposing army to avoid midfield and more advantages to refuse flanking the Imperial player (before the AV12 could simply rotate to present the AV12 facing and make it harder to flank them).
Conclusion
An AV12 wall isn't a necessity of an effective Imperial Guard list of course but even five midfield based Chimeras can support the ranged firepower of an Imperial Guard army very well. Take that into the 10+ range and opponent's have a lot to chew on in midfield, sometimes to an extent where they can't overcome it. With that in mind this really cannot be done without the Chimera due to it's ability to also take infantry units into midfield which can shoot from the top-hatch and provide double the targets to your opponent. Add in the two heavy weapons the Chimera handles which can be customised to provide duality to whatever unit they are transporting (usually the multilaser + heavy flamer combination is best for some S6 firepower and an anti-infantry template up close) and the wall in midfield can be very aggressive or defensive depending upon the opposing army.
In the end, having the added mobility to an Imperial Guard army is necessary, whether or not you aim to move those aspects of your army into midfield and present an AV12 wall to your opponent is inconsequential as 5th edition cannot be won without mobility (unless you table your opponent every game). Whilst there are other ways to move the Imperial army out of their deployment zone the Chimera is cheap, durable, adds firepower and is effective at what it does and it becomes very hard to overlook for many Imperial Guard armies.
