Seems like the Union wanted to get revenge for bad treatment of Union prisoners held in Confederate POW camps.

Of course the Union blockaded any food or medicine from entering the Confederacy.

They were trying to starve out Southern civilians by burning crops, killing or taking farm animals and destroying any food they found. How could they expect the Confederacy to have food to feed their POWs?

The Union had plenty of food and starved the Confederate POWs as retribution.