Google Maps for Android has a feature by which shaking the device opens a "send feedback" dialog. This baffles me. What could possibly be the rationale behind this? Why would one want to shake the phone to send feedback? One does not want to send feedback often, and having a gesture dedicated to it (specially which can get activated accidentally on a bumpy road - this has happened to me) seems weird. Shaking is a rather awkward gesture, and having anything assigned to it seems weird. Why does this feature exist?
I don't think anyone can answer "why" except perhaps the designer of the feature, who may have been trying an experiment.
Apple uses "shake" on the iphone as an "undo", which I think is an homage to the Etch-A-Sketch, and is only slightly less obscure.
The overall problem is "keep these things off the interface because you don't always need them, but when you do, you need them fast." So smart phones and tablets have embraced this practice of hiding functionality behind meaningless gestures that are almost impossible to discover by accident, yet are poorly documented anywhere else. It's extremely poor design and UX, but it's a tradeoff they chose to keep the hardware from being cluttered by "undo" buttons and other such interface objects. So it's a compromise: they provide a needed fast access mechanism for some fairly frequently used features at the expense of UX.
And in their defense, once learned, the gestures are not difficult to master. It's the learning process that is unpleasant, but that's a one-time effort. So for now, they stay.
Out of a set of bad gesture choices, I place shaking as among the worst. But then again, I'm not the billionaire getting rich off smart phones, so what do I know?