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human
(WarHammer40K) Why are Titans so darn important? Furthermore, what are they, exactly?

Whenever I watch a WH40K cutscene or any gameplay from a game, all they ever seem concerned with are these giant, walking mechs. 

They always seem to talk about them like they're the greatest things since sliced bread, but never really expound on it. Can anyone explain precisely what they are, how they're made, why they're so relevant to the Imperium, how they're even used(how do they use them on other planets if they're so big?), and why they always need to be protected?
gpt
Titans have a number of valuable attributes.  One, they are avatars of the Emperor in his form of the Machine God, and are thus holy icons.

Two, they are bad ass motherfuckers.

Let us put in easily compared perspectives of Epic 40k: (A relatively accurate simulation of warfare commonly played amongst the lazy nobility of certain upper hives)

**A Leman Russ battle Tank:**  
Speed, 25 CM (a cm is about the length of a battle tank, so go with 10 meters per cm).  
Combat effective range:  45 cm  
Firepwer (in the form of its main gun, as well as twin sponsoon heavy bolters), 3.   
Armour of a 6+.
Assault Capability: 1 (this is for directly assaulting enemy positions)  

**A warlord titan** (the essential main battle titan and backbone of the titan legions):  
Speed: 15 cm  
Armour: 6  
Void Shields: 6.  These are rechargeable layers of defense which are powered by the fusion reactors.  
Damage Capacity: 12  
Armament:  
32 Firepower (or an assortment of specialized and very nasty other weapons).  
Assault Capability: 30 (this is for directly assaulting enemy positions)  

So you see, a single warlord titan has the firepower of 11 battle tanks, but also has the damage capacity of 12.  However, it carries rechargeable shields which allow it to shrug off hits that would devastate a tank battalion.  But the real strength is that when assaulting defending positions, a single warlord titan is **30x** more effective than a Leman Russ.

Now, these are just stats from a simulation.  But you can see why its such an asset on the battlefield.  Its only real weakness is that its a larger target and less maneuverable.

Titans are difficult and complex to manufacture.  but they can last for thousands of years if properly maintained, and recovered from the battlefield.  Only the Adpetus Mechanicus can manufacture them, and only on the best of forge worlds.  They're deployed via special dropships which are large enough to carry them.  They come in a variety of sizes and models, but there are four general lines used by the imperium (and even these will vary by forge world or construction date).

**Warhound Titan:**
Warhounds titans are the smallest titan, and are used primarily as scouts for the main Titans.  They're 25% faster than the other titans, but only mount a small amount of shields.  They are, however, still deadly machines and carry the equivalent firepower of four Leman Russ battle tanks.  


**Reaver Battle Titans:**
Reavers are the "mediums" of battle titans, and the smallest that are capable of carrying anti-titan close combat weapons (the most effective way to take a titan out, is at close range with another titan).  Their maximum firepower is equivalent to 8 Leman Russes.

**Warlord Battle Titan:**
As previously discussed, this is the most relied upon battle titan.  Their  additional shields give them great staying power.

**Imperator Titan:**
The largest titan in imperial service, this beast is incredibly rare and incredibly dangerous.  They mount, conservatively, the equivalent firepower to 20 Leman Russes, but it is difficult to quantfy giving the sheer insane power of their Plasma Annihilator and Hellstorm Cannons.

In combat a Titan represents the ability to concentrate the firepower of a battalion of tanks.  Their strength in close assault is unmatched, and only the insane will defend a position against an approaching Titan.  But their true greatest strength is their ability to directly engage the Titans of the enemy.  The imperium isn't the only force which can bring these constructs to battle.  And the best way to stop a titan, is always with another titan.

edit:  Added some commentary explaining the source of the information and fixed a few typos.