That was the thinking of the time, capture the capitol and you win the war. "On to Richmond" and that sort of thing. Grant and Sherman were the ones who turned it away from that. Grant realized that to defeat the confederacy you had to remove their power to make war, take out their army. Sherman realized that in order to win you had to remove the enemy's ability to wage war. Food and supplies were as vital as soldiers and munitions. Take away those and you defeat him as surely as on the battlefield. That was why he marched to the sea.