The AT-AT is absolutely a troop transport. With shields against orbital bombardment being the norm in any military matter the Empire anticipates, AT-ATs serve as a way to land troops outside of the range of an area shield, penetrate that shield, and deliver troops to the target location.
That said, they are not purely troop transport. They also are heavily armed, not just armored, and not with strictly anti-personnel weaponry. They have armament that seems befitting of an armored fighting vehicle (such as modern tanks), and they pursue missions which seem tactically similar to those of modern armor.
Specifically, in the assault on Echo Base, the troops carried by the AT-AT walkers are doubtless those which we then see in the initial ground assault. The ground troops move in to secure the main buildings and capture the base. The AT-AT walkers, however, have another mission: they target and destroy the shield generator. This sort of mission, a precision strike upon a fortified target, is one which an armor unit would be well suited to.
With all of that said, the AT-AT was a poor weapon. It had a maximum speed of 60 km/h, could carry 40 troops, and had a very high center of gravity. Like the Death Star, it was large, over-engineered, and designed more to inspire fear than to be an effective weapon of war.
Compare the AT-AT to the earlier AT-TE. The AT-TE could carry a platoon of troops, was capable of scaling vertical cliffs, and didn't require the troops to embark or deploy from a platform 10 meters up. The AT-AT was less a weapon of war than a weapon of terror. They worked, but an up-armored and up-gunned AT-TE, with its lower profile and increased deployment flexibility, would probably have worked at least as well.