The goal is to build a keyboard filter — something that sits in between the keyboard and the main computer, and provides macro capabilities.
There's also the goal of building a USB Rubber Ducky, but that's for another day, for more malicious purposes. The keyboard-filter is useful for benign purposes of macros. It could also be used as a keylogger, but again, this page is for more benign goals.
These macros must be BUILT-IN via firmware, inside the keyboard, since many of the computing environments I work in don't allow installing ANY software, which excludes nice options like AutoHotkey. The upside to this is that they work in any OS, not just Windows or MacOS.