They implemented this protocol on the Micro, and used the Keyboard library to create a HID device. The final product is an adapter for NeXT to USB, which allows for the old keyboards to be used ...

[Rampadc]’s keyboard adapter ... the old keyboard becomes one of the best portable keyboards in existence, and probably a bit cheaper than the official Lenovo USB-bound ThinkPad keyboard.

In short, many old PCs simply weren't made to interface with USB devices at all, and some simpler adapters may not even work properly, depending on keyboard or system. An all-in-one compatibility ...