FREE black slaves?? union?

How did freeing the salves help preserve the union? Lincolns ultimate goal was to preserve the union, in fact in one of the speeches he gave he said that he would do ANYTHING to preserve it. If that meant NOT freeing the slaves he would do that. But, he believed that freeing them would help preserve the union.. why? wouldnt that make the confederate states more mad? less willing to join the union? i mean..they supported slavery..they wouldnt agree with free black people. hoow would this make them change their mind and be a part of the union again?