LogFAQs > #948056173

It puts the weapons systems higher off the ground, allowing them greater range and a better field of fire, at the expense of being a better target. Of course, with enough armor and/or shielding, that becomes moot (AT-ATs on Hoth, for instance)

Probably even more important is the effect to enemy morale. You'll hear an AT-AT coming before you can see it, and you'll see it much further than you could a ground tank. And such a large, imposing vehicle plodding unstoppably towards you would be enough to scare the crap out of many soldiers, especially if it's heavily armored and your weapons are having no effect.

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Minutus cantorum, minutus balorum,
Minutus carborata descendum pantorum.