Depending on size, seems like it would it serve as a mobile forward operating base. lower guns covering for infantry, maybe armor piercing rounds. Upper guns are for anti air/long range artillery bombardment.
Possibly artillery; the size and sturdiness of the legs would support a massive amount of recoil, and it doesn't look fast enough to be a frontline combat unit. Alternately it could be a multipurpose mech, with a hardpoint on top for mission-specific turrets such as anti-aircraft or missile battery.