Why was the Sherman March unnecessary?

Answer 1

It was not unnecessary. It shortened the war by months.

It was, however, the very opposite of what Grant had ordered Sherman to do.

Sherman's orders were to destroy the Army of Tennessee, wherever it might be. He had consistently failed to do this, and when he captured Atlanta (also not part of Grant's plan), the Army of Tennessee got out safely and headed North.

Fortunately, Sherman carried high credibility with Grant, who eventually endorsed this new idea of punitive raids on the Georgia farmland that was the bread-basket of the Confederacy.

As for the Army of Tennessee, that was completely routed in December by George Thomas at Nashville.