One of the most important goals of King's "I Have a Dream" speech was to remind the American people that the goal of true freedom for African Americans had not yet been realized, and he began the speech by using lines alluding to Lincoln's Gettysburg Address, an opening designed to link Lincoln's Emancipation Proclamation of 1863 to King's speech on the Washington Mall in 1963. Implicit in the opening sentence of King's speech is that, despite the passage of 100 years, the promise of the Emancipation Proclamation was still, in most respects, a promise, not a reality.
Among other things, King reminded his audience that the Declaration of Independence extended a promise of freedom to all Americans, and he used an economic metaphor to get his point across: he said that African Americans were there to cash a check, a promissory note created by the Declaration, and it was up to the American people to make sure that check was not returned marked "insufficient funds." His use of the "check" metaphor was particularly effective because it brought the abstract concept of seeking freedom into the real world--many people in his audience and beyond the Mall, white and black, understood the contract implied by a check presented for payment.
On the whole, the importance of King's speech was that it brought into focus, particularly for the masses of people not engaged in the struggle for civil rights, that the struggle was a work in progress.
Martin Luther King's "I Have a Dream" speech, delivered to thousands of observers on the steps of the Lincoln Memorial in August of 1963, essentially established King as the national spokesperson for the civil rights movement. King had long been recognized as an important leader in the movement, but this speech, thanks in part to its power and elegance, and also to its setting and visibility, brought King the acclaim of white liberals. Later that year, he was TIME Magazine's man of the year, and he won the Nobel Peace Prize in 1964. It was in many ways the crowning achievement of King's career, mostly it helped to raise pressure on the federal government to pass a Civil Rights Act, which they did in 1964.