One big reason: It gave the North an additional, powerful reason to fight and win the war.
Additional reasons: It gave the Union Army another source of soldiers, and it kept foreign powers from allying with the Confederacy.
Historical context/details:
President Abraham Lincoln issued The Emancipation Proclamation as an executive order on January 1, 1863. The executive order declared freedom for slaves in ten Confederate states in rebellion against the Union. It also allowed that freed slaves could join the Union Army to fight for the cause of reuniting the nation and ending slavery. As summarized by the Historical Society of Pennsylvania, "The Proclamation broadened the goals of the Union war effort; it made the eradication of slavery into an explicit Union goal, in addition to the reuniting of the country."
While Lincoln personally was strongly against slavery, he had to tread carefully in his role as president and commander-in-chief. The Emancipation Proclamation was carefully worded in order to retain the support of four border slave states, which remained in the Union though they were states that permitted slavery, were Maryland, Missouri, Delaware, and Kentucky. Lincoln wanted to keep those states loyal to the Union cause.
The Emancipation Proclamation was also a way of blocking foreign support for the Confederate cause. According to the American Battlefield Trust, "Britain and France had considered supporting the Confederacy in order to expand their influence in the Western Hemisphere. However, many Europeans were against slavery." Britain had abolished slavery in its territories in 1833. France had put a final end to slavery in its territories in 1848. So when Lincoln issued the Emancipation Proclamation on January 1, 1863, it also served as a foreign policy action to keep European powers out of the US Civil War, according to Steve Jones, professor of history at Southwestern Adventist University.
