Desk accessories were tiny applications that you started from the Apple menu, and they were common in the System 6 era and earlier. (System 7 through 9 allowed you to put pretty much anything in the Apple menu.)
These systems didn't support multitasking (or, in the case of System 6, it could be disabled) so an alternative means of having concurrently open programs had to be developed. This mechanism was through writing the second program as a device driver.
Of course, this had trade-offs. Which is why it wasn't done for major programs. One trade-off was that DA development was more complex since it was a device driver and it had to communicate with the currently running application. There were probably resource limitations too, since most DAs are small.