The Emancipation Proclamation was largely symbolic. It freed the slaves in most of the areas under rebellion, not throughout the entire country, and almost all of the places covered under the order were controlled by the Confederacy at the time of its signing.
What it really did is signify that the Civil War was being fought for more than the suppression of a rebellion or the preservation of the Union. The Emancipation Proclamation made it clear that the abolition of slavery was a key objective in the war. With the Emancipation Proclamation, Abraham Lincoln basically married himself to emancipation as a condition of victory.
It doesn’t matter that the actual order was virtually toothless. The symbolism behind the Emancipation Proclamation helped lift the spirits of an oppressed people, helped establish a lofty cause for a nation exhausted by combat, and ensured that foreign countries would resist supporting or giving international recognition to the Southern Confederacy because of the North’s intention to abolish slavery.