Historical postcards are printed to commemorate events such as war, social problems, expositions, parades, coronations, politics and so on. These cards offer much to the serious collector in the way of increased value. This is a wide open field with much to offer anyone interested in twentieth century history. Often this type of card was made of a real photograph with few copies being offered for sale. This is especially true of disaster cards depicting floods, fires, wrecks, etc. Often the historical significance of a card comes form the message written by the sender.
A Google search reveals links to many museums and library collections of postcard images related to a specific location. So pick your location and/or topic and begin your collection!