Belgium had previously been a province of Luxembourg. However, through various major events in Europe (not the least of which was the Napoleonic Wars), the signatories (Great Britain, Austria, France, Prussia, Russia, and the Netherlands) established the independent country of Belgium, and gave Britain this special role.
It was an important document, especially in its role in expanding World War I. The German government asked the British government, in 1914, to ignore the "scrap of paper" compelling Britain to guard Belgium's neutrality. Britain refused, and eventually, Germany invaded Belgium. At that point, British Prime Minister Herbert Asquith declared war on August 4 of the same year.