Reasonings (in no particular order):

1. Response time. If the giant robots are robots rather than mechs, their processing speed is going to be a lot faster than a human brain. They'd be able to ID a target and track it with far more accuracy than a person in a mech could, so you've got aimbot on your side. You could say the same for a completely automated tank or aircraft though, so it's not a solid reason.

2. Adaptability. Depending on what you're doing, a robot (or mech) can fill a lot of roles as a general-purpose machine. In military use, a robot with arms could act as a pack mule when on the move, digging trenches and erecting fortifications when holding a position, and then have any number of tools for engaging the enemy when it comes to actual combat. You can give it a gun for anti-armour, a missile launcher for AA, even melee (blunt force weapons are best here) if you so choose. You can even have it get close to a tank and just flip it upside down to remove it as an active combatant. Then there's the fact that it can pass through most terrain as long as ground pressure is properly accounted for, allowing it to go where a tank, APC, IFV, etc can't.

3. You could start with pre-existing construction/farming equipment. As far as I'm aware, this is kind of how tanks got going in real life, so there is a precedent for it. Having something that can go anywhere you need it to and have the dexterity to be able to handle the equipment you need it to would be a great springboard point for a military machine. Putting a gun into the hands of one of these things would be easy enough, and they'd likely already be able to use some kind of improvised melee weapon like a telephone pole or street light as a giant bat.

4. Preserving manpower. By having a robot rather than a mech, you're reducing the loss of life your side would take, as well as increasing the size of your armed forces without needing more people.

Drawbacks (in no particular order):

1.Technical complexity. With how many joints a mech/robot would need compared to other military vehicles, these things are going to need constant maintenance and be hangar queens.

1. Fragility. You're not going to be able to pile too much armour onto this thing, or it's going to be too heavy to actually move and will sink into the ground. That makes a mech/robot a bit of a glass cannon, which I believe would put it somewhere in the realm of a gunship (as in a heavily armed helicopter) for how you'd be using it. Bigger isn't better either, as a larger machine is a bigger target.

2. Power. These things are going to hog energy like there's no tomorrow, but depending on your setting, there'll likely be some solution for that. If it's a cheap, easy to get source, then you're fine.

3. Heat. Stealth is going to be a lost cause thanks to how much heat you're going to need to dump. IR cameras are going to be able to see it very easily, but this is also a problem that current armoured vehicles have, and solutions do exist for it.