Back in the mid-60s when rock was changing and the new hippie counterculture was being born, there was no Internet, and television and newspapers were for squares (unhip, uncool folks) who were a part of the establishment. So bands and promoters of dances, festivals, and other hippie gatherings used the old idea of public postings, but done in a new way.
Artists created a new style that originated in their personal experience using legal and illegal mind-expanding substances – in fact, the basic and original meaning of the word “psychedelic” is “mind expanding.” These creations were meant to advertise the upcoming event, but were done in a colorful, surrealistic way that made many of them difficult to read.
This feature of the design was partly intentional – you had to work at reading it and pay attention – and partly the way the style developed. Early examples mimicked antique circus posters, but they soon became more relevant to the artists' and audiences' experiences. Not surprisingly, many of the classic ad posters from the era are today valuable collectibles, and there is a large market for psychedelic posters of this early kind in good condition.