Up to the Emancipation Proclamation (EP), the aims of the Union under Lincoln had been restoration of the Union - to bring the Southern state back into the Union even if slavery were left intact.
With the EP, the North objectives included the freeing of the slaves. The South now knew - there was no compromise with the North. The South would have to fight to the end to maintain slavery (and their entire economic system). Slavery would either live or die with the outcome of the war.
This from Wiki:
"Slaves had been part of the "engine of war" for the Confederacy. They produced and prepared food; sewed uniforms; repaired railways; worked on farms and in factories, shipping yards, and mines; built fortifications; and served as hospital workers and common laborers. News of the Proclamation spread rapidly by word of mouth, arousing hopes of freedom, creating general confusion, and encouraging thousands to escape to Union lines."
