Traditionally (and still very common today) Chinese, Japanese and Korean were written vertically. This is not yet possible with QT.
This is not just rotating the entire text about 90%, but instead the writing starts at the upper right corner, going down in a line to the bottom of the page, continuing in the next line left to the previous line. The individual characters are not rotated, but are instead stacked on each another:
Also, some characters look slightly different when written vertically, e.g. 、。「っ and so on. I think, this is done via the “vert” substitution table in the fonts.
Vertical writing is also extremely necessary for Mongolian, which can only be written vertically. In this, the writing differs a little bit from CJK writing, because it starts at the upper left corner.
All in all, this is really a necessary feature.