Why did shermans march take place?

Because he was worried about his long and vulnerable supply-line, and believed that he would not be able to pursue and destroy the Army of Tennessee in difficult mountain country, as Grant wanted.

If he turned South-East and conducted a punitive raid on the Georgia farmland, ending with the liberation of Savannah, he would be able to live off the land.

This March to the Sea shortened the war by months at almost nil casualties.