Why do holidaymakers still bother sending postcards? Now that we can relay instant text messages and snapshots on our mobile phones, the postcard seems like the definition of a redundant technology. By rights, it should be going the way of the telegram.

But the point of the postcard is that it is a material object, with its own elaborate rituals of sending and receipt. Tourist etiquette demands that it is purchased at the place it depicts, is rendered personal by a handwritten message on the back, and is posted (rather than handed over) to someone back home, even if it arrives later than the person who sent it.

So the postcard is the epitome of what linguists call phatic communication: a message with no inherent content, sent for its own sake and simply saying “hello, I’m here and you’re there”. The most touching item in Martin Parr’s cult book of Boring Postcards is a postcard of Reighton Sands Holiday Village, on which someone has scrawled the words “our caravan” in blue biro, next to one of about fifty identical-looking caravans.

The compound adjective, “picture-postcard,” to describe a scene of exaggerated prettiness, is misleading. Postcards have an inclusive, non-judgmental aesthetic. Often cheaply and locally produced, they have consistently expanded our definitions of the picturesque. In postcard land, as Parr’s collection shows, the Chiswick flyover and the Arndale Centre in Crossgates are as worthy of attention as the Taj Mahal and the Eiffel Tower.

The messages written on the backs of postcards are also a social leveller. Before the arrival of the postcard in the late nineteenth century, there were Byzantine rules about how to open and sign off a letter, depending on one’s social status and familiarity with the sendee. It was bad manners to send a short letter, because the recipient often had to pay for the postage. But even the barely literate could write a brief postcard message, and they did not have to worry about whether to put the effusive “yours sincerely” or the more formal “yours truly”. The postcard message was the equivalent of today’s text message: non-elitist, informal, and laid back about spelling and syntax.

In his book The Post Card, the philosopher Jacques Derrida argues that postcard messages are a strange mixture of the public and private, circulating “like an open but illegible letter”. The message is written casually and can be viewed by anybody, including the postman; but the sender often writes in private codes and assumes a body of knowledge only shared with the sendee.

Tom Phillips’s anthology, The Postcard Century, which relates the history of the twentieth century through thousands of cards, is full of these kinds of semi-decipherable messages. The banal evasions of postcard language – “wish you were here,” “having a lovely time,” “we saw this and thought of you” – can hint at the much larger world of happiness or misery behind them. Senders of cards rarely produce the expected or appropriate responses to historical events, as their everyday anxieties intrude into era-defining moments. “I have been gardening all this week,” declares one 1911 card which carries a photograph of a condemned man in an electric chair. Another message from Berlin just after the fall of the Wall in 1989 complains to the recipient that it is “definite thermal undies weather”. The postcard message, as Phillips says, “bumps into history as a ball on a pin-table hits or misses, by hazard”. If all the postcards sent this summer were collected into an archive, their mundane, barely legible messages about missed flights and dodgy weather would eventually make for an equally gripping historical record.