Quite: "Honest question here. Why would I need to download a driver for a USB device?".
Basically USB provides connectivity to the device, but doesn't necessarily provide the driver to actually use it.
Many USB devices, such as keyboards, mice, data drives etc. Follow a USB standard, defined for that device type (the 'class'). For example keyboards and mice should be using the USB HID class (Human Interface Device).
The idea being you plug any keyboard or mouse in (and other devices) and USB, and so the OS, knowns how to work with it automatically. A driver is still used, it's just that it's included as standard with the OS, and so is basically transparent to the user. (It's also why you can plug USB keyboards into devices like an XBox One, smart TVs, Sky box etc. and they actually work).
But for rare devices, not covered by a specific class (recent examples being VR headsets for example), as mentioned in the article, they may not be covered by one of the standard USB device classes, and so need a specific driver to be installed to be used (the computer knows a USB device has been plugged in, but not how to use, or at least not properly, it till the driver is installed).
In the early days of USB, many USB device needed it's own driver, these days it should be fairly uncommon to need to install a separate driver..
See here for a list of the standard USB classes:( I know, wiki!)