Lincoln issue the proclamation to score points with the British. He calculated that if he framed the war as being about slavery, the British would not openly support the South.
And a good strategy it proved to be. States rights over slavery was a foolish issue to make a stand. Making the issue clear to the Brits kept them from selling out principles for King Cotton.
That was surely part (but only part) of his calculation and it worked magnificantly. There were many powerful people in Britain who could see that united, America would eventually eclipse them as the world's leading power and they desired to see the union shattered and weakened and were using their influence in the British government to force them to take the side of the Confederacy. By making the war a moral issue, Lincoln cut the legs out from underneath that faction.
But Lincoln in the summer of 1862 also still hopped that he could break the Confederacy and get at least some of the less radical Southern states to return to the Union using a 'carrot and stick' approach. That, didn't work.