system
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human
[Star Wars] Why did the empire choose the AT-AT design?

I was thinking about design and history of the Empires military technology. And while there is a clear lineage to the republic era equipment some of the choices puzzle me.  

In particular the AT-AT and AT-ST vehicles.  Both use walking systems commonly found in terrestrial species for locomotion in order to carry their weapons systems and protect the crew.  

Unfortunately this design choice has created incredible vulnerabilities.  As well the size, moving parts and energy expenditure seems out of control relative to the weapons systems they are carrying. 

So why did these designs win out over say a hover tank or other smaller systems that would be far cheaper to produce, easier to transport, far less vulnerable to ropes rocks and slippery terrain.  Two significantly smaller tanks could provide the same effective fire power of the AT-AT.
gpt
The Empire has a fundamentally different military doctrine than the Republic, especially compared to the Clone Wars era Republic military. While the Republic was in the business of efficiently waging war against comparable enemies, the Empire rules over a period of relative peace and as such it's military focuses on all-purpose equipment to enforce rule over inferior targets. The best example of this is the ISD II. It has fewer fighters than a Venator, lacks the missile launchers of the Victory and Malestrom, fewer troops than the Acclamator, is a 5th of the size of the Mandator, and has fewer ion canons and point defense weapons than the Imperial I. In other words, it's not as well adjusted to any of the more specific roles of these other ships and overall less optimized for warfare. But this is because they aren't ships designed for symmetrical war. Rather, they are all-purpose space superiority vessels, a Swiss army knife of a capital ship which excels nowhere but can be called on to do anything. 

The *all terrain* vehicles of the Empire have a similar doctrine. They have obvious weaknesses, but that's okay because they're used for force projection and putting down terrorists, not waging war. Repulsor craft are great, unless you want to deploy somewhere with microgravity or with dense vegetation and so on. The height of the AT-AT and AT-ST also give the advantage of increase line of sight and simple intimidation. 

Also, mechanical locomotion will actually be a much smaller energy drain compared to repulsors on a vehicle of the same weight, and the vehicle can stand if power is lost.