The shift controls are designed to do for a projector what a "rising and falling back" does for architectural photographers: enables you to keep the various elements parallel to each other and to the vertical. For a photographer, this enables a "virtual" -instead of real - tilting of the camera up to get a tall building in the image without the "keystone" distortion produced by tilting the whole camera.
If you visualize the keystone that caps an arch at top center (also called the "capstone"), it's broader at the top than it is at the bottom. That's where the term "keystone" comes from. By keeping the film and lens vertical and parallel to the building, but moving the film up and down behind the lens, the building can be brought into view without introducing that distortion, which would make the top and bottom of the building different widths, and the sides no longer parallel.
That's what both the mechanical and electronic shifts are modeled on - you keep the imaging panels parallel to the screen while moving the image up and down or side to side behind the lens, avoiding the geometric "keystone" distortion that tilting the projector would introduce.
That's why you shouldn't tilt the projector.
End of pedantry (for now).