Why did General Sherman attack the civilian infrastructure

(basic facilities such as roads and bridges) between Atlanta

and Savannah?

Because he was expressing total war. Total war is a war method used today it is destroying everything a soldier needs to survive like food weapons and clothing factories. Total war is like taking out the home life of the military. Sherman was the first to do this in American.

General Sherman attacked the civilian infrastructure between Atlanta and Savannah to suppress the Army of Tennessee

Explanation

Sherman referred to his march from Atlanta to Savannah as "to make Georgia Howl".

His move was entirely tactical and considered him as the first to do this in America.

He destroyed farms, burned crops and wrecked rails to destroy the sources of the army that stood against him.

By this, he brought the war to the end, rather than proceeding to fight resulting in zero casualties.