Why Shermans march to sea take place?

Answer 1

The "March to the Sea" was the name given to the phase of Sherman's campaign between Atlanta (a major rail and manufacturing center, but not at that time the capital of Georgia) and Savannah (a major seaport).

Atlanta was a target because it was a major rail and manufacturing center, at least by the standards of the South. After taking Atlanta and effectively destroying it, Sherman turned toward Savannah which in addition to its port facilities was one of the ten most populous cities in the Confederacy.

However, it's remembered more for the utter devastation Sherman left in his wake. Sherman intended to utterly destroy Georgia's capacity to make war, and was also "living off the land" (by confiscating or foraging supplies as needed) since he was operating behind enemy lines and cut off from the main Union forces.