According to the Library of Congress Webcasts site:

"The 100th anniversary of the official use of American personal identity tags, affectionately known as 'dog tags,' recently passed without fanfare. Dog tags are highly personal items to warriors of every service and to their families as well. Each dog tag carries its own human-interest story. The acts of receiving the dog tag, hanging it around the neck, and feeling it against the body constitute a silent statement of commitment. The tag itself individualizes the human being who wears it, despite his or her role as a small part of a huge and faceless organization. While the armed forces demand obedience and duty to a higher cause, dog tags, hanging under service members' shirts and close to their chests, remind them of their individuality. They bring comfort and help calm the fears of soldiers facing death: 'I do not want to be forgotten; I do not want to become an unknown."