At the turn of the century, written communication was still the main form of communication. The papers of political leaders such as George Washington, John C. Calhoun, Theodore Roosevelt, and "Pitchfork Ben" Tillman are filled with historically valuable correspondence and memoranda documenting in great detail the philosophies and attitudes of these powerful leaders.
In modern times, we've seen a tremendous growth in the volume of the documentary record being created and, at the same time, a decline in the value of that record in documenting the beliefs and activities of these individuals. More and more important communications occur in person, over the telephone, and in other ways which do not produce a physical record which can be preserved and made available to future generations of scholars.
Oral history provides historians and archivists with a mechanism to supplement the documentary record and fill gaps within the collections we preserve. We all know that oral history presents some problems, chiefly the vagaries of human memory and the natural inclination to remember and present things in a favorable light. But, in the absence of other records, personal reflections form a valuable and increasingly popular historical resource.