What did the Emancipation Proclamation do?
First, we should acknowledge what the Emancipation Proclamation did not do. It did not immediately result in freedom for enslaved people. This is because Lincoln, in the Proclamation, declared freedom for only those slaves who lived in areas under Confederate control. In the border states and even in some areas occupied by Union forces, the Proclamation pointedly did not grant enslaved people their freedom. But this does not mean that the Emancipation Proclamation was not an important document, and to understand why, we have to look a few things that it actually did.
The Emancipation Proclamation was framed as a wartime measure, and Lincoln justified it by asserting his powers as commander in chief. One thing it did was to allow African-American men to enlist in the Union Army. Almost a quarter of a million black men seized this opportunity to fight for the freedom of enslaved people, and Lincoln claimed shortly before his death that they helped turn the tide of war.
Another thing the Proclamation did was to redefine the purpose of the war, which had previously been conducted primarily to maintain the Union against secession. While no slaves were freed when Lincoln issued the proclamation, the question of whether slavery would persist after the war was resolved. The war became a war to end slavery. This had important consequences. First, it meant that the war would be a war to the death, as the South was fighting to protect its social and economic structures, based as they were on slavery. It also meant that Great Britain, which had long contemplated recognizing the Confederacy and intervening to negotiate an end to the war, abandoned this idea. Having abolished slavery in the British Empire in 1838, the idea of intervening in a foreign conflict on behalf of a Confederacy explicitly fighting to defend slavery was no longer politically tenable.
So the Emancipation Proclamation, despite not granting immediate freedom to enslaved people, was a document of profound importance.