It's not an AT-AT (All Terrain Armored Transport), it's an AT-ACT (All Terrain Armored Cargo Transport). Per StarWars.com (emphasis mine):
A larger version of the standard combat AT-AT, the AT-ACT walker features a dedicated cargo bed for the transportation of heavy building materials or combat munitions. It was deployed at major Imperial construction projects, such as shipyards and sprawling research installations.
The hull of AT-ACT is hollow, allowing it to transport materials, munitions, or stormtroopers in large amounts.
There is a picture that I found, fram an early draft of Rogue One: visual Dicitonary (hat tip to @Valorum):
Cropped by me, original here
And there's another one (one more hat tip to @Valorum):
AT-ACT has laser cannons and armour as well, making it only slightly inferior to AT-AT in terms of sturdiness (if you don't count longer legs) and firepower.
I'd say it's a pretty good balance between a cargo hauler and a tank.