Answer: Serial Ports
Before the ubiquitous USB port—now used for everything from printers, game pads, smartphones, and everything in between—computer users had to deal with much bulkier and less effective cabling and data ports.
The most widely used peripheral cable and port combination prior to the introduction of the USB standard was the serial cable and serial port. Parallel ports saw widespread use for printers and scanners, but for everything else—including dial-up modems, communication with UPS systems, serial mice, and thousands of various specialized peripherals—serial ports did the heavy lifting.
Compared to USB connections, serial connections come off as severely underpowered: USB 1.1 sported a 12 Mbps bandwidth compared to serial’s 115 Kbps, you can link multiple USB devices to a single USB hub/bus as opposed to serial’s one-connection-per-port limit, USB can provide power for the peripheral over the cable whereas serial could not, and serial ports were neither “plug and play” (PnP) friendly or hot swappable.
Whatever grievances modern users might have against USB, the ease with which they can plug in a multitude of devices to their computer without having to add expansion cards, boot down, or otherwise inconvenience themselves should more than make up for them.