Why did Lincoln give his speech at Gettysburg?

The Gettysburg Address, given by Lincoln over four months after the conclusion of the Battle of Gettysburg, represented more than just a dedication to the National Cemetery at Gettysburg. In his iconic address, Lincoln not only reiterated ideas expressed in the Declaration of Independence, but he also extended these ideas to African Americans and redefined the war as a struggle not only for the preservation of the Union and the ideals on which it was founded, but also as a struggle for human equality. 
The Battle of Gettysburg marked a turning point in the Civil War, ending Robert E. Lee's march into the North. Lincoln's speech must be considered in the larger context of the war and in light of his previous actions in framing the meaning and course of the war. From the start of the war, Lincoln asserted that the war was about preserving the Union. However, after a Union victory at Antietam in 1862, Lincoln chose to issue the Emancipation Proclamation, expanding the Union's goals to freeing slaves and preventing European intervention in the war. With the Gettysburg Address, Lincoln again expanded the Union's purpose in the war, from freeing slaves to ensuring equality and a "new birth of freedom." In remembering the fallen at Gettysburg, Lincoln simultaneously asserts the importance of the war and of again creating "a more perfect union," this time one in which a government dedicated to equality for all men "can long endure." 

The Battle of Gettysburg was a decisive moment in the American Civil War. It marked a key defeat of the rebel Southern armies by the Union Army and a turning point in a civil war that had already caused the death of thousands of men on both sides—here, the war began to swing in the Union's favor. After Gettysburg, those who had fallen in the battle were reburied in what would later become the National Cemetery at Gettysburg. Lincoln was invited to give a speech on the occasion of this cemetery's consecration. The speech he ultimately gave was very short, around two minutes in duration; the photographer, who had expected a lengthy speech, had not even finished setting up before Lincoln had finished and left the stage. Afterwards, Lincoln lamented that he had not spoken for longer and had not, in his eyes, done justice to the solemnity of the occasion. He was sure, as the speech notes, that few would remember the events of that day; however, the Gettysburg Address has gone down in history as one of the greatest American speeches ever made.