For quite a while, the parallel port was the only external high(-ish) speed port on PCs, not only used for printing, but also scanning, networking (with a yellow LapLink cable, or, if you wanted to get really fancy, a Xircom Pocket Ethernet adapter that almost did 10Mbits/s) and even storage (various weird MO contraptions, but also a regular hard disk with almost 1MB/s read performance...).
The history in the linked article is quite comprehensive, and touched on the slightly-incompatible (EPP, ECP) standardization efforts for bidirectional use. When USB finally came along, that made things a lot more convenient.
Out of desperation, I once soldered a bunch of resistors onto a parallel port connector to create a crude DAC converter so I could get my PC to output "high quality" audio. The results were pretty good! I could play MOD files as if I had an actual sound card.
Early USB was terrible on both hardware and software support. It took a long time for most people to stop using parallel. Honestly, the only thing I ever used USB for before the USB2.0 era was computer mice.
I usually ended using the PS/2 adapters because the USB mice wouldn't work when you first plugged them in, but they did if you used the PS/2 adapter, and then I'd just leave them that way.
My first MP3 player (the Rio PMP 300) and digital camera (Canon Powershot 600) both used parallel ports for data transfer. Plenty of other weird things used
the port, Autocad’s licensing dongles come to mind. It was really used for some versatile functions!
As a young kid I remember developing this conclusion that Macs must be better because ours had a SCSI port and the devices were just better and faster than the parallel port devices of our PC.
The history in the linked article is quite comprehensive, and touched on the slightly-incompatible (EPP, ECP) standardization efforts for bidirectional use. When USB finally came along, that made things a lot more convenient.