The Postcard

Since the first picture postcards appeared in 1869, these small cards have managed over time to become a testimony of the past, a top-notch historical document which covers a time spectrum of close to a century of our history, and a cultural object which symbolizes the development of an era that covers the major recent events and many aspects of daily life. The First World War, in its magnitude and its consequences, has been one of those historical chapters of our recent history and the graphic arts became a mechanism for the expression of the events that happened during it. It was during the war that the popularity of photographs, posters, prints and, of course, postcards became more widespread. Through the analysis of more than 10000 postcards, consulted mostly through the webpages of the most important international documentation centers, we seek to take a close look at this historical development and to focus on the role they played in the conflict as they became one of the main media used in the dissemination and propaganda of the battle.
Citation: Researchgate.net
www.researchgate.net/publication/317775266_The_postcard_as_a_historical_document_A_visual_approach_to_the_First_World_War

Also, a potted history of the postcard, from the late 1800's to the present is given here.

 www.arthistoryarchive.com/arthistory/photography/History-of-Postcards.html

Briefly, the periods can be broken down into:

Private Mailing Card Era, 1898 - 1901
​The Golden Age, 1901 - 1907
Divided Back Era, 1907 - 1915
Early Modern Era (White Border), 1916 - 1930
Linen Card Era, 1930 - 1945
Modern Chrome Era, 1939 - Present

The evolution of the postcard continues today, perhaps at a much slower rate (Due to things like Instagram, mobile phones, etc.,) but there is still a significant business within the printing business for such item.

Historically, postcards were important social documents. When people could not visit places easily, and where the average person could not take a photograph, due to prohibitively high equipment costs, it was natural to use the postcard - not just of a way to send "tourist" images, but those of a social documentary nature. Many millions were sent during the first world war, for instance. 

Today, there are multiple avenues where the Postcard exists. As discussed, there is a large element of "Tourist" cards, but there are also social documentary styles, and "Art" style - almost mini portfolios of various artists.