If Donald Trump loses the election and doesn't concede, it won't violate the U.S. Constitution. But it would break a tradition of concession that dates back more than a century and has achieved quasi-constitutional status. And like most enduring political customs, its value goes beyond graciousness: It helps ensure the continuity of government and offers a legitimating assist to democracy itself.
It's a matter of interpretation exactly when the practice of concession began. Thomas Jefferson drafted a letter of concession to John Adams even before the election of 1796 was complete, in which he said he expected to lose and warned Adams to be careful lest Alexander Hamilton cheat him of his "succession by a trick." In the end, Jefferson didn't send the letter, but instead gave it to James Madison, who passed on its contents indirectly to Adams.
In 1861, Stephen Douglas, the defeated Democratic nominee, called on Abraham Lincoln to say that "partisan feeling must yield to patriotism." (Al Gore quoted Douglas in his own concession speech after the Supreme Court ruled in Bush v. Gore that George W. Bush was the winner of the 2000 election.) With the preservation of the union in the balance, Douglas's comment was especially important — although as it turned out, woefully insufficient.