"Important" is a pretty subjective term. To me, as a historian, it shows how much worse things could be than they are now. We, as a nation, have survived the unimaginable, and Lincoln's short message in the address is one of hope. Remember what was going on in 1863, even though the Union had just won Gettysburg, there was still almost two full years of bloody war left, and Lincoln himself was not the least bit assured of re-election.
As a political document, the "Gettysburg Address" is not necessarily relevant other than from an historical perspective. However, it is a beautiful piece of literature, and even though it was written for an entirely different group of soldiers and nation, it's qualities do, as another poster has mentioned, make it timeless and powerful.
Lincoln's "Gettysburg Address" moves beyond honoring the dead to creating new tasks for the living. This is done through a standard eulogistic technique. Lincoln never really specifies the “great task” remaining. The speech of the speech moves beyond a focus on the nation to a focus on the world. Because of its emphasis on fundamentals, and by not focusing on what had happened, but rather on what it meant, the speech achieves a timeless quality.
I think that the Address is important in a rhetorical sense. How many Presidents or politicians write their speeches today? Because so many speakers--political or not--depend on cues, teleprompters, and other "helps," Lincoln's ability to craft the speech so quickly and appropriately should still amaze modern readers/listeners. Besides its content being important (remembering the great loss our country suffered), the speech presents many examples of effective rhetoric.
It depends on what you mean by important.
In one way, it is not important. It is not a law that people have to follow. It does not give anyone any rights and it does not take away any rights. It is simply a short speech that a president made almost 150 years ago.
In another way, it is important. It is one of those statements that define who we are as a people. It is part of our heritage of democracy because of its idea that American soldiers die so that government "of the people, by the people and for the people" can survive on Earth.