Why were the border states so important to Lincoln in reuniting the nation?

Maryland was the capital,first and foremost, and besides the border states being full of resourceful and tactical rivers, they had an exceptional populous. Had they been 'allowed' to fight for the Confederacy,it's likely the Union wouldn't have won the Civil War, as the South was already lower on population than the North, and having the Border States would have given them a substantial boost.