Since we're talking about uses for cassette tapes, here is one use that was quite technical, was available to only a few members of a minority group (blind), and didn't last very long.
In 1979, TeleSensory Systems, a Mountainview, California-based technology company that manufactured and sold products for the blind and low vision community, came up with the VersaBraille. What the company did was tcreate a product that allowed blind people to use cassette tapes as basically braille notetakers, and I actually got to work with one of the company's products while I was doing a summer 1988 volunteer internship at Channel 10 in Phoenix (I can't remember if it still had the KOOL-TV callsign at the time.)
Anyway, the product looked like a standard cassette player/recorder and you could play standard cassette tapes in it in the standard way. However, it also allowed you to, on higher-quality Type 1 cassette tapes (it didn't work on Type 2 or Type 4 papes) press abutton that transformed the cassette into, essentially, a braille notebook. You could then write notes in braille using a braille keyboard and then compartmentalize those notes into however many different categories you wanted. If you played the portions of the cassette tape with the braille notes on them with a standard recorder, all you heard were a bunch of random beeps.
Here is a website with more information and a picture of the VersaBraille. Enjoy!