Apple2History.org reminds us of a time before floppy disks when the cassette tape was the standard way of writing and retrieving data from an Apple ][.
The earliest Apple II owners did what most of the microcomputer hobbyists of the day did – they used the lowly cassette to save the programs they wrote, or possibly to load software that was purchased. And even after the Disk II did appear in 1978, it was still $495. Although this was less costly than floppy disk drives for other micros of the the day, it was still about one third of the cost of the entry level Apple II ! For many who pioneered the use of the Apple II, it was simply not affordable to get that expensive (though highly desirable) Disk II drive, at least not for a couple of years. From 1977 until around 1982, there were a significant number of software titles that were sold on cassette, because it was the most affordable way to use the computer.
I love that Apple still keeps Apple ][ Cassette interface instructions in its knowledge base.