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.
The Civil War campaigns were the complex of military operations aiming to reach a strategic objective like: seizing a strategic point or city, destroying or capture an enemy's army. The most important of them were: the Peninsular Campaign; the Vicksburg Campaign; Lee's Maryland Campaign; Lee's Pennsylvania Campaign; the Chattanooga Campaign; the Overland Campaign; the Petersburg Campaign; the Atlanta Campaign; the March to the Sea Campaign; Hood's Tennessee Campaign; the Appomattox Campaign.
The Union Army under General Sherman won the Atlanta campaign in the Civil War. (That happened after the President of the Confederacy, Jefferson Davis, replaced the Southern Commander, General Joseph E. Johnson, with General Hood. Many in the South considered that to be the dumbest decision of the Civil War!)
One of the most significant battles of the US Civil War was in July of 1864. This was a Union victory by General Sherman in the battle of Atlanta. Atlanta was an important Southern city, and was thought to be safe from Union attacks. The capture of Atlanta by General Sherman, paved the way for his famous scorched earth policy of his March to the Sea campaign.
Based upon common computation, about 10,000 battles, large and small were fought during the Civil War, of which the most important (battles and Campaigns) had been more than ten. They were: First Bull Run; Peninsular Campaign; Seize of New Orleans; Shiloh; Second Bull Run; Antietam; Fredericksburg; Chancellorsville; Gettysburg; Stone's River; The Final Campaign against Vicksburg; Chickamauga; Chattanooga; Atlanta Campaign; Overland Campaign; Siege of Petersburg; March to the Sea Campaign; Appomattox.