Scorched Earth
Like a swarm of locusts sherman's army went through Georgia. They lived off the land, because the large army couldn't have gotten enough supplies otherwise. His scorched earth policy was obvioudly controversial, but he had the support of Lincoln and Grant, and it really hurt Lee. If it weren't for Sherman, Lee would have been resupplied, and reinforced by Johnston's Army of Tenessee. It would have been much harder for the Army of the Potomac to finally beat Lee. Sherman's strategy was especially effective because the South didn't have nearly as much resources as the North, so when he captured the big cities in the South it hurt more than if the North lost a couple industrial centers, because they had plenty of them.
It also had an affect on morale, in the north and south. His victories gave Lincoln the momentum he needed to stay in office, and pass the controversial emancipation proclamation. And in the South people would have been upset that their amries couldn't stop Sherman, and that they can't comunicate with other towns.
Atlanta Burns
When the garrison of Atlanta fled they actually already burned a lot of it themselves, buildings, and warehouses that could be useful to Sherman's army. Before going in and burning Atlanta Sherman sent a letter telling the citizens to evacuate. 30% of Atlanta was burned altogether, Sherman's soldiers, some of whom were drunk, went beyond what they were told to burn and lit private homes and businesses. As they were watching the city burn Sherman told his officers " since they have done so much to destroy us, and our government, we have to destroy them."
Within one month Atlanta elected a new mayor,and restored mail service, and in 1865 all their railroads were already working again.
"You cannot qualify war in harsher terms than I will. War is cruelty, and you cannot refine it; and those who brought war into our country deserve all the curses and maledictions a people can pour out. I know I had no hand in making this war, and I know I will make more sacrifices to-day than any of you to secure peace." excerpt of Sherman's letter to the people of Atlanta prior to burning it-from http://www.rjgeib.com/thoughts/sherman/sherman-to-burn-atlanta.html