The Gettysburg Address is perhaps the most famous oration by an American president; other contenders are Franklin Delano Roosevelt's Pearl Harbor address and John F. Kennedy's inaugural address. In the case of all three speeches, the moment in history in which they were delivered influenced their enduring effects on Americans.
Lincoln traveled to Gettysburg by train to honor the fallen soldiers, numbering over 7,000, in the midst of the Civil War. The Gettysburg battle had the largest number of casualties of any battle to that point and remained the largest battle by the time the war came to a close. The impact of the battle on the American psyche inevitably lends to its enduring legacy.
The opening and closing lines of the speech are the most often quoted:
Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
This underscores how young the new democracy was and the necessity of preserving the Union for all. The final clause, "that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth" has been repurposed throughout history to defend or oppose perceived threats to the nation and to democracy in general.
Finally, the delivery of the address took place on the field where so many died, where bodies had been piled just a few months prior. Photographs from the battle were some of the first wartime photos any American had ever seen. Therefore, Americans had a sharper idea of the cost of human life in the war. Lincoln's words were meant to honor the dead and comfort the living, and they are still used for that purpose.