It's all about symbolism. Edited.the symbolism
People also invest emotions in symbolism. Edited.
The Dred Scott decision was symbolic and so was the North's decision to totally ignore it. Lincoln sending the fleet to Fort Sumter and then calling up 75,000 militia was pure symbolism. John Brown's Raid on Harper's Ferry was purely symbolic.
Symbolism is an essential part of politics . . . and war and violence are political tools.
Napoleon used to say that "Men will fight long and hard for a bit of colored ribbon."
A flag in the 1860s was probably the most powerful symbol of all. Consider this Medal Of Honor winner, for example:
William Harvey Carney became the first African American soldier to be awarded the Medal of Honor for a heroic flag rescue. Born as a slave in Norfolk Virginia, Harvey escaped to the North and enlisted in the 54th Massachusetts Volunteer Infantry. During the regiment's assault on Fort Wagner (depicted in the 1989 film Glory), Carney took the regiment's flag from its fatally wounded bearer and marched on. Though he received multiple serious wounds, he carried the flag throughout the battle and eventually made his way back to Union lines. As he handed the flag to a fellow soldier, he said "boys, I only did my duty; the old flag never touched the ground!"