You could pick them up for a nickel or so, or if you were on holidays, you could get them for free from almost every Motel 6 and HoJo's in the country. They cover every subject, every location, every hobby in the world. It was a form of damn near free self-promotion that even the most cash strapped mom and pop shop could afford. There must be billions of them out there. Like matchbooks and paper coasters, they were meant to be used and then thrown away. The very definition of ephemera.
Those old throwaways can be amazingly valuable. If you are of Chinese heritage or had a family member who sent postcards from Mainland China at the turn of the last century, some of these vintage cards can sell in the thousands of dollars. Postcards are still collected by a huge number of people. These "deltiologists" are avid, rabid specialists. From Greyhound Racing to National Parks to specialty buildings, to slave depictions, there are thousands and thousands of sub categories. Why collect?
From the Toronto Postcard Club
"Because they are fascinating pieces of history. Initially, people are drawn to vintage postcard views that have a connection to their lives. Home towns, occupations, a genealogy connection, places visited, and so on. Then, from their interactions with other members and collectors and from their search for that next treasure, their postcard knowledge is expanded. They learn about the impact of postal regulations, the different eras of the postcard craze, the different types of postcards, early Canadian photographers and publishers, and more. It’s a hobby that inspires more research and understanding of the view and the times represented in a 3-1/2″ x 5″ picture from a century ago."
Yes, postcards may seem oldschool with the advent of the computer, and the crash and burn of the physical penmanship skills that we no longer learn, but for those diehards who love to collect history, this field is wide open. Easy to find, cheap to buy, millions of images. What's not to love.