Say I was a Traffic Control Gentleman who wished to implement some form of number-plate reading technology, for whatever purpose*.
My first experiments with a standard lens parallel with the sensor were unsuccessful because vehicles moved through the plane of focus before I could photograph them.
So, being a clever photographer, I closed-down my aperture to deepen the depth-of-focus. Except then I need a slow shutter to compensate for the loss of light, and the vehicles became blurs...
How to solve this dilemma?
Tilt the lens with respect to the sensor, laying the focal plane parallel with the road and using the focus control to set its elevation above the surface. Then I can keep my nice wide aperture and hence fast shutter whilst not 'wasting' the focal plane pointing up in the air...
All exposure settings identical, all that changed was the tilt of the lens. Mamiya 80mm Macro at f/4.
Actually upon examination I didn't quite have the plane parallel with the 'road', as you can see it slopes-up slightly from the near field and the nearest '410g''registration' isn't as sharp as it could be.
* subsitute tins of pears for vehicles in this example. Part of your five-a-day.