If you are controlling an AT-AT, you are only allowed to move its head-mounted turrets. You can move these around to fire at Revel infantry and enemy players that are on the ground. Players are also able to call in an orbital strike.
The senior producer for Star Wars: Battlefront, Sigurlina Ingvarsdottir, talked to Eurogamer and told them that DICE was forced to keep the AT-ATs on-rails in order to avoid “game breaking” consequences. Ingvarsdottir also talked about the lack of space battles that are in the game and talked about more details on the Battle of Jakku DLC.