Lincoln issued the Emancipation Proclamation for several reasons. First, it deprived the south of potential labors by inspiring slaves to escape. At the same time it also bolstered Union ranks with new recruits when runaways from the south came north looking for an opportunity to fight. By the end of the war more than 1/10th of the entire Union army was African-American.
Lincoln also issued the proclamation as a means of redefining the war morally. By saying that the aim of the war was to free the slaves as well as keep the Union together, Lincoln ended any hope for support from Europe that the Confederates had left. No European nation would side with the south once the Emancipation Proclamation had been signed.
Lincoln also worded it carefully. It said that the slaves were free in the states who were in rebellion, but neutral states like Delaware and Maryland weren’t affected. Technically, it didn’t free a single slave until 1865, but the symbolic meaning to African Americans was powerful!