Probably only hipsters use them nowadays, but cassettes were formerly a popular way of copying audio. However, it wasn't just audio. Beginning from the early 1970s, people would use audiocassettes for data storage because they were cheaper than floppy disks at those times.
Most home computers came with a built-in cassette drive. Of course that much popularity caught some attention. In some countries, including the United Kingdom, Poland, Hungary, and the Netherlands, some radio stations began to broadcast computer programs so that the listeners could record and load into the computer!
However, as by 1981 floppy drives became more common among new products, cassette tapes lost their popularity but reigned supreme in music industry until the late 1990s.