Why was the atlanta campaign important to the civil war?

In itself, it was not important.

Grant had changed the rules, and declared that he was not interested in occupying territory or capturing famous place-names. He just wanted to destroy enemy forces.

Atlanta was no longer an important military objective. But Sherman had not succeeded in destroying the Army of Tennessee, and this was an election year, when Lincoln could easily be voted-out, with disastrous consequences.

So the capture of Atlanta became politically important - "Atlanta Falls" made a heck of headline, and it helped to restore Northern morale, along wth some conveniently-timed victories at Mobile and in the Shenandoah Valley.