You can also use one hand to hold the machine and the other to write. Handwriting technology permits free-hand drawing as well. For parse trees, flow charts, org. Use it for rough drafts. Eventually, some pattern matcher will probably be able to convert a free-hand drawing into structured graphics.
Keyboards tend to be too noisy for meetings, classes, libraries, etc.
Handwriting is quiet. Some people don't type. Some people have only one hand. Perhaps, there is less stress on tendons from handwriting than from striking keys. On the other hand, I do not get cramped fingers from typing.