Why was general grant important during the civil war?

Grant was not a good general. Tecumseh Sherman and even George Thomas were better Union generals.

But as Abraham Lincoln said, Grant fought whereas McClellan mostly sat.

So, Grant's biggest strength was that he was a fighter and that he was able to think clearly during the battle.

Grant's biggest weakness was complete lack of reconnaissance. He was taken by surprise at Shiloh, but more inexplicably he was taken by surprise at the Battle of the Wilderness. By 1864, the Union had cavalry that was the match of that of the South. So, Grant should have been able to know at all times the strength and disposition of Lee's forces. Also, the battle took place in almost the same location as the Battle of Chancellorsville. Fool me once, shame on you. Fool me twice, shame on me.

I believe that the US Army has come to value qualities of U.S. Grant and have conscientiously created such generals as George Patton and Douglas MacArthur. There are similarities in the way MacArthur was surprised by the Red Chinese and the way Grant was surprised in the Wilderness.

Grant was only a very average General.