Why did Lincoln write and deliver the Gettysburg Address?

The occasion of the Gettysburg Address was the dedication of a new National Cemetery for fallen Union soldiers at the site of the Battle of Gettysburg. Lincoln's purpose in writing and giving the speech was not just to pay tribute to the war dead, but to make it clear that their noble sacrifice was ultimately made for a worthy cause.
Good must come out of bad. And the soldiers who so bravely gave their lives on the field of battle died to further the cause of liberty, the great American tradition bequeathed to us by the Founding Fathers "four score and seven years ago." (Eighty-seven years earlier, in 1776 with the Declaration of Independence).
ButLincoln says that the heroic fallen soldiers at Gettysburg didn't simply validate the American understanding of liberty; they inspire us to resolve that "a new birth of freedom" will arise on American soil. Lincoln refers here, somewhat obliquely, to his own changing understanding of what liberty means in the context of American history and politics.
The concept of freedom devised by the Founding Fathers was largely negative, that is to say freedom from—particularly freedom from British rule. Lincoln signals in the Address that the new birth of freedom will be a positive freedom, a freedom to do things. He also alludes that "government...for the people" will involve an increased role for the Federal government in the protection of Americans' inalienable rights, particularly those citizens recently set free by the Emancipation Proclamation.
http://www.abrahamlincolnonline.org/lincoln/speeches/gettysburg.htm