Why did general sherman attack the civilian infrastructure
(basic facilities such as roads and bridges) between atlanta
and savannah?

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.