AT-AT walkers are used by the Empire in a manner that is tactically identical to how a modern day army would use a tank, in that they are used as shock armour with powerful main guns which will punch through entrenched positions. (No combined arms tactics were used in the Battle of Hoth, but that's a different question altogether)
However, AT-AT walkers have a number of (rather lethal) drawbacks and no advantages that I can see compared to tanks.
- They have footpads in place of treads, which greatly increase their ground pressure, and which will cause them to easily sink as compared to treads
- They have a high center of gravity and can be easily tripped, which was exploited in the Battle of Hoth. (granted that this was canonically presented as an oversight, albeit a rather stupid one, since most martial arts strongly emphasise lowering your CG.)
- They are slower than a modern main battle tank despite their long legs, which one would normally expect to grant additional mobility (max speed 60km/h on flat terrain, compared to 72km/h on flat terrain for the M1A1 Abrams). This is exacerbated by the fact that AT-AT walkers are using fusion power cells as compared to diesel engines.
It would seem that it would make far more sense for the Empire to remove the legs and mount the main chassis on treads, which would make the vehicle far more effective. Why then, did they not do this? It clearly was not the case that imperial Engineers lacked imagination, as they did produce wheeled armoured vehicles.
Possible answers which I find implausible as advantages:
1: AT-AT walkers are tall, and therefore have a surveying advantage over the battlefield / have a better vantage point to fire from
Orbital satellites can provide the observation, and in any case air/orbital support should always be called in, especially if fire support from above is required.