The Emancipation Proclamation was a major document in American History. Slavery was an issue in the time of the Civil War. The Emancipation Proclamation was more of a rallying cry for the North than a document to free the slaves of the Confederacy. Lincoln stated that, "My paramount object in this struggle is to save the Union, and is not either to save or destroy Slavery." (Lincolns letter to Horace Greeley). This statement lays testimony that ending slavery was not a priority for Lincoln.
Before the issuing of the Emancipation Proclamation the North was having little success on the battlefields. This document conserved the momentum of the North from the battle at Antietam. Lincoln thought it would be inappropriate to issue the document without success in the war. If he had issued it earlier than Antietam it would have looked like the only way he could win was by causing the slaves to revolt. (Ace of Spades Cartoon) By issuing the Emancipation Proclamation on January 1, 1863 Lincoln was able to boost the moral cause of the Union.
Another aspect of moral cause is that Northern armies combined. Lincoln helped make blacks become somewhat more equal. The North was fighting to help free them. "Don't kneel to me. That is not right. You must kneel to God only and thank him for the liberty you will enjoy hereafter." (The Great Emancipator, cartoon) Therefore, the North was not just fighting for the Union they were fighting for the Southern black slaves.
The Emancipation Proclamation was not aimed at freeing all slaves or else the border states would have had to free their slaves. The North depended on these border states heavily especially since the capitol was in a border state. The moral cause was the biggest reason the North won the war.