I'm wondering how you expect a Giant to be as well armored as a battleship. The Giant would barely be able to move around if only because having a meter of armor around their joints would impede them too much. Also Naval Guns tend to be long, the Iowa class guns were 20m in length so they would have to use a shorter version which would limit their projectile velocity and thus effectiveness. Even tank guns are a sizeable portion of the Giant's length, making these guns unwieldly. Even if you assume a Giant can somehow carry all that despite the square-cube law he's going to be rather burdened by that. It's far more likely he's carrying tank armor and tank guns, although I would think they would be more like AFV's than tanks.
Anyways, reasons to use infantry next to your Giants with the unlikely armor+armaments:
* Scouts
* Communication (with radio's and such)
* Repairing and maintaining gear of the Giant
* Anti-armor support with missiles and such. Tanks would still be a threat despite battleship armor because of the squishy Giant inside. 347
A HESH shell would do squat to a battleship, but a shockwave going through the brain/lung/bloodvessles is going to wreak havoc on the Giant and probably kill him.
* Anti-Giant support. Whether using armor-piercing or a superheated jet of a HEAT round or just HESH weapons, the use of infantry-based AT weapons are going to be effective against the joints of the Giant where the material has to move with the joints and can't be as thick. This can incapacitate Giants if not kill them with a shot to the neck for example.
* Soft terrain traversal/terrain scouts. These Giants even without armor will weigh well over 100 tons, with armor it's likely they'll exceed 500 tons (because that armor is going to weigh more than the Giant). This isn't going to be nice travel when you hit soft ground where they'll likely sag into the ground, get stuck and starve unless some heavy-duty cranes get brought in. Cranes that just like the Giant need to be careful with what ground they stand on even before they lift a 16m Giant in full battleship armor.
* Building capture. You can't capture buildings with tanks, and you can't capture buildings with Giants. 347