Why did sherman order to march to the sea?

Answer 1

Chiefly because his supply-line had become so long and vulnerable that he could not conduct the big mountain campaign that Grant had ordered.

Noting that there were very few Confederate troops in Georgia, he decided to turn South-East and make punitive raids on the farms and railroads, to ruin the Southern economy, destroy civilian morale, and help starve the Confederate troops in the field.

This woud end with the liberation of Savannah, one of the blockade-runners' favourite ports.