Answer: Divide the upper and lower South.
General Sherman's "March to the Sea" was a march of a fighting force of around 60,000 men, going from Atlanta to Savannah, Georgia (a distance of 285 miles). It was intended as a show of force, to intimidate. They did not burn everything in sight, but did burn the houses and barns of any people who tried to resist them or fight back.
In addition to intimidation, Sherman also believed that the destruction caused by his march would effectively divide the South in two. As described by William Stroock, writing for the Warfare History Network (December 31, 2018): "Sherman’s grand idea: ... He felt that from Atlanta he could march to the sea and rip out the heart of the Confederacy. The march would create a dead zone that split the Confederacy in two."
