Some people just like tiny keyboards. Moreover, keyboards are quite fun to build and typing on a keyboard you built yourself is satisfying : you have so much choice regarding components.

I think that very tiny boards are not really used by their owners (see the Gherkin board for example), but they still are a lot of fun to build. Just don't take it too seriously.

The OLKB Planck seems to be a nice tradeoff between usability and space efficiency.

I think you're right about English tiny keyboards, but Japanese ones seem more potentially useful to input kana.

Some very small layouts can in fact be used, and quite effectively. I have a co worker who types using stenography (and develops with it as well) and the absolute minimum keys he really needs is around ~25.

He also absolutely dumpsters me at typing speed, be it normal text or special character heavy code, and I type around 90wpm with an ergodox and 110wpm with traditional layout.

Ted?

I would guess that they can be used for writing Japanese (Tokyo keyboard meetup..) via kana, and choosing conversions to kanji suggested by software, perhaps with those thumb buttons.

It's just a guess.

I also would like to know about if there are any ergonomic health research about these keyboards. Which one is the more efficient? Or the one more healthy?