More important, arguably, than gaining enthusiastic Black military support was the commensurate depriving of the South of that same support. From te beginning of the Civil War slaves were either employed in direct military-assistance tasks such as digging trenches, building breastworks and the like, or working on plantations – either way they represented a considerable contribution to the Southern war effort and economy. It was this contribution that Lincoln was determiined to undermine.
Slaves had, of course, voted with their feet from the outset of the War long before the issuance of any Emancipation Proclamation. If Union forces were anywhere near them they, with greater and greater frequency, made their way to the safe side of Union lines where they were used [exploited?] in performing roughtly the same military-assistance tasks. They were, to use the parlance of the time, contraband.
What had been a steadily increasing stream became a torrent once news that Lincoln had freed the slaves spread like wild fire in the South.
The South, which suffered from a population disparity with the North of 5 to 2, could not afford the loss of over a million slaves. On the other hand, the Emancipation Proclamation in effect left the South helpless to do anything to stop this particular population hemmorhage.