Most computers could use the 'remote' line of the cassette input to start and stop the tape under software control. This allowed for primitive forms of data swapping but because it couldn't fast forward or rewind, it could only replace loaded data with data later in the tape.
One interesting use, for the time, was interactive literature. Data would be loaded in, the student would see graphics on the screen to go along with audio being played from the cassette.
The Coleco Adam computer included a high speed tape deck that could ff and rwd, making it a simple form of random access data drive. Too bad they fucked up some other aspects of the computer, causing the tape drive to be destructive to data.