I use it as a qwerty keyboard but... I use the extra functions keys as "super" and "hyper" in addition to ctrl/shift/alt. I love japanese keyboards because the space bar is tiny so the modifiers left and right of the space bar are easy to reach.

The "super" modifier is only for shortcuts related to my window manager (switching virtual desktops / arranging windows / launching programs etc.).

The "hyper" modifier is for my custom macros/functions (which I call from Emacs).

It's nice because these additional modifiers never clash with any other shortcuts.

And as I type this I see in this thread I'm not the only one using a japanese keyboard this way.

Now... In this day and age I probably should get a custom keyboard or make my own keyboard but... Years and years ago it wasn't that easy to make your own keyboard so back then it made sense to go for the laziest thing that would provide me with more modifiers and with "easy to reach with the thumbs" modifiers (thanks to the small spacebar). And the lazy thing to do was simply to use a japanese keyboard.