I'm going to have to disagree with Mr Plinkett on AT AT's in Rogue One.
If you look at the Empire's military (around the time of the Original Trilogy, which is the only time worth looking at), the Star Destroyers are Destroyers (but they don't Destroy Stars, the Death Star doesn't even do that, hurr durr). They're fucking warships. Designed to kill anything on the sea, and the Empire's preferred military tactic is to use cruise missiles to blow up anything that stands against them.
They've also got Tie Fighters and Tie Bombers. These are basically aircraft. And the Star Destroyer doubles as an Aircraft Carrier.
The AT AT's are tanks. All Terrain tanks. They call them Armored Transports, but with the guns on them being able to take out shield generators, they're clearly tanks that pull double-duty as armored transport for Stormtroopers or cargo or whatever. The Empire didn't anticipate using them on Hoth, they just found a shield generator that blocked cruise missiles and bombers, so they shrugged and sent in tanks, which they happened to also be carrying in the Star Destroyers. They're not walkers because snow, they're walkers because All Terrain.
It makes sense for a major Empire base with a planetary shield and Star Destroyers in orbit to have some military stationed on the ground. There could be a garrison. There could be tanks. There could be parked Tie Fighters. There wouldn't be a Star Destroyer, but that's only because Star Destroyers are too big to land. Nobody would expect the tanks or Tie Fighters to actually
be used inside of the shielded safe zone, and Stormtroopers would probably be tanning on the beach if their supervisors didn't come along every ten minutes to kick their asses, but soldiers can really get ready in a hurry once the bombs start exploding.
And besides the logic of commonly being able to find parked tanks hanging around somewhere in a military base far from the front lines, the AT AT's in Rogue One were shown with their doors open on their massive cargo holds, meaning that the AT AT's at this base were actually being used for their named purpose, carrying cargo.
The complaint with AT AT's in Rogue One, like the complaint with X Wings in Rogue One, ties into the larger complaint that Rogue One has a fanservice problem. Rogue One is inherently somewhat fanservicey, but AT AT's and X Wings were used in a slightly more fanservicey way than they needed to be, and when surrounded by more glaring fanservice elements, it becomes hard to distill where one layer of fanservice ends and another begins, and the whole movie falls apart, dismissed as fanwank. Mr Plinkett would know this if he studied more animu.