I have been dreaming of this feature for ages, and then I bought a tablet PC a few days before it get implemented in Krita when I do not need it anymore But it is still an extremelly cool feature, that was started by Lukáš Tvrdý and finished by Dmitry Kazakov.
When drawing in real life, like on paper, it is very common to rotate the paper to be able to positionnate your hand to optimize the control on a curve. Unfortunately, rotating the tablet does not work, the brain seems unable to map the hand gesture with the screen content. The only solution is to rotate the content, in older release, you would have had to use the transform tool, but then it introduce quality loss for each rotation, and now, there is a feature that rotate the canvas without affecting the quality. The pan tool allows to select the rotation, by shift-clicking.
In combination with the infinite canvas and the possibility to mirror the canvas, Krita makes sure artists get the best drawing experience.