He issued it as a political move to unstablize the unity of the south. He had previously warned that if the states in rebellion did not stop rebelling, he would issue an edict that would confiscate both land and slaves that landowners held in the rebellious states.
When a suitable amount of time passed, and no state conformed, he on his own, and without the approval of Congress declared the Emancipation Proclamation. It was not a law exactly, but it was not challenged in any court -- especially the Supreme court. Slaves in occupied lands were immediately freed as a result of this edict. And if any escaped, they too were freed.
It was not a complete solution to the slavery question, but it was a good strong beginning.
