the answer is commemorate the grounds of the battlefield
Many soldiers died at Gettysburg. I believe it was the bloodiest battle. Lincoln wanted to pay tribute to those who had died and also wanted to let those who were left to clean up the mess and put the pieces back together, to know that the battle was not fought in vain. The battle was about keeping the Union (North and South) together as well as fighting for the rights of others.
Hopefully this helped and good luck.