Lincoln decided to issue the Emancipation Proclamation for a number of reasons. First and foremost, he knew that declaring an end to slavery would keep Europe out of the war. By re-defining the Civil War as a war to preserve the Union AND end slavery, anti-slavery European countries could not come in on the side of the south on purely moral grounds. We now know that Confederate diplomats were very close to negotiating Europe’s entry into the war and that this proclamation was part of the reason these negotiations never worked out!
Also, the proclamation would deprive the south of badly needed labor since slaves would no doubt try to escape north.
He carefully worded the proclamation so that it only slaves in the Confederacy were freed. While this didn’t actually free a singles slave, it did keep the boarder states from joining the Confederacy and symbolically moved us closer to the ideals set forth in Jefferson Declaration of Independence, "All men are created equal!"