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.
The answer is 'to commemorate the grounds of a battlefeied'
There was a commitment of the battlefront graveyard in Nov. 1863 following the July fight and President Lincoln was urged to address at the commemoration. Lincoln was ordered since he remained the leader and commander-in-chief of the country.