The purpose of the, "march to the sea," was to frighten the Georgia's civilians into abandoning the confederates cause. Total war was brutal and destructive, it hurt the south Morale, and made it impossible for the south to fight at full capacity. Once Atlanta was captured that was a symbol to even the most loyal southerner that they couldn't win the war
During his the march, he didn't destroy towns, he only destroyed houses and barns of people who fought back. Also on the march he destroyed plantations and crops, and pillaged the stores of food. After burned plantations he basically forced slaves to join the army because they had no where else to go. Also he destroyed cotton gins, and lumbers, anything that helped the confederates economics.