Before USB ports became standard on PCs, several devices connected through a serial port. They're obsolete in many ways, but ...

When a byte is received over serial, the individual bits are set on the individual digital lines connected to the printer’s parallel port. At this point, the strobe line is pulled low ...

Because there was only one serial and one parallel port on the computer, the key acted as a pass-through so that other keys and peripheral devices could connect. In fact, the nickname "dongle" was ...