Lincoln said of emancipation that "he had given it much thought and had about come to the conclusion that it was a military necessity absolutely essential to the salvation of the Union." Why did he believe this?

The quote in the title is from the Diary of Gideon Welles, recounting the carriage ride in July of 1862 in which Lincoln floats the idea of a proclamation declaring emancipation to Welles and Seward.

Why did Lincoln (and others) believe that emancipation was a military necessity?