According to President Lincoln's words from the Gettysburg Address, why is it important for the Union to win the Civil War? What is at stake in the war? Why was the sacrifice of the people who died at Gettysburg so important?

Lincoln said the best way for the living to honor the dead is to continue to fight. Their father, grandfather, uncle, brother, etc, gave their lives to end slavery. They have to fight to win the war because the outcome of the war is what they were all fighting for.