Hello there!
The main reason on why President Abraham Lincoln chose to ussie the emancipation proclamation during the war instead of waiting until the war was over was because it looked like they were going to lose to war, so he issued it in hopes of encouraging the soldiers to fight on. During the war, the North (the side that President Lincoln is on) were losing so many battles. Due to them losing so many battles, it would make them have a lesser chance of getting what they wanted, and that was abolishing slavery. Lincoln was getting so worried, that he issued the emancipation proclamation while the war was still going in. He did this so he could boost the spirits of the Northern soldiers up so they could keep on fighting to victory. If he didn't do this, the North would've probably lost the war because there was nothing to encourage them to keep on fighting.