At the time Lincoln issued the proclamation it was about a year after the Civil War started and the Union needed a bigger reason to fight the rebellion of the south and their slavery. At first, the Lincoln and his soldiers were fighting to squash a rebellion and keep the Union a whole. But as time went on, this reason for fighting just wasn't enough and when the Emancipation Proclamation was issued it was a rally for all northern
soldiers and people to support and win the war, who were abolitionists and Republican party supporters.