Why was the march to sea campaign important?

It put out the clearest possible signal that the Confederacy was too weak to survive - far more so than Grant's slow progress at Petersburg.

By despoiling the farms and killing the livestock, Sherman struck a terrible blow at civilian morale, while also bringing the Confederate armies another step closer to starvation.

It did not really matter that Sherman had failed in his mission to destroy the Army of Tennessee. That army essentially had nowhere to go, and would soon be pulverised by George Thomas at Nashville.