Why does Lincoln claim The Emancipation Proclamation is necessary?

Lincoln claims The Emancipation Proclamation is necessary to weaken the Confederacy's ability to sustain the Civil War by depriving it of labor and resources. He views emancipation as a strategic measure that aligns with the Union's moral obligation to end slavery. Additionally, he believes that freeing enslaved people will bolster the Union's resolve and support for the war effort, ultimately contributing to a more just and equitable society.