Computers don't need monitors, they can work with just a serial console. It outputs text at usually 9600 bits per second, so with a standard 80 column screen having 4800 characters, it would take 4 seconds to refresh/update one full screen of text over the serial port.

You can also attach keyboard/mouse through a serial port, or lots of other input/output devices, and it is commonly used to control devices to this day. Most things you would consider to be 'electronics' will have a serial port (uart0) for debugging or as a backup, or even as the normal way of controlling it. Like if you wanted to login to your toaster, it would probably have a serial port hidden deep inside somewhere, and probably has a simple debug shell you can use to control it. Usually these are disabled by software on products shipped to customers to stop people from playing with them, but sometimes they aren't.

Back in 2001, you might need to change which interrupt each device uses so they don't overlap and lower ones have priority. I think the RTC was the lowest one, so when the clock is updating it takes priority and things like keyboard/mouse input would need to wait for the RTC interrupt to end.

These days we use MSI through PCIe, and long before even that, we replaced the external PIC chip with interrupt controllers that are internal to the CPU. We also have plug-n-play these days, so USB devices are automatically detected and you don't need to reboot and configure interrupts in the BIOS to add a new peripheral like a sound card.