Why did Sherman's March To The Sea happen?

Answer 1

Because he was worried about his supply-line, and told Grant that he would not be able to pursue the enemy into difficult mountain country, as ordered.

Instead he wanted to carry out punitive raids on the Georgia farms and railroads, living off the land as he went. This would wreck the Southern economy and help to starve the Confederate armies in the field.

As Sherman carried credibility with Grant, this was eventually approved, and it turned out to be a spectacular success, shortening the war by months at almost nil casualties.

Answer 2

Because General Grant wanted to wage total war on the Confederate States. General Sherman destroyed everything in his path on his march.