Secondary custom keyboards are terrific if you're doing something technical — like editing music or video, or even playing a video game. You can assign a key to run a command or a macro you use repetitively.
Over at Reddit, "RonaldMcWhisky" describes how he took a USB cash-register keyboard — the Wincor Nixdorf TA85P, bought for 30 euros on Ebay — then plugged it into his computer and found he could easily read the keypresses using Python. This makes it nicely hackable and repurposable as a custom keyboard, because, as the folks at Hackaday observed, cash-register keyboards are big, chunky, reliable, and designed to be easily re-labelled:
Cash register keyboards have a number of advantages over a standard QWERTY design. They have big keys, the keys can be labeled, and the keys can be ordered in a way the user wants. The hardware is also cheap since cash registers are everywhere.
Very fun idea — I might try this myself. Though I think I'll have to find a different model of USB cash-register keyboard, because when I checked ebay right now this particular model was selling for almost $300.