I think to some extent, it may be to accentuate the feeling of a car on the limit. A touch of suspension compression would naturally give an angle to a shot, but maybe to show this more, or even to give the impression of speed or 'effort' a tilt can perhaps make the viewer that this is happening.
Some of it may be also to try and use the angle as a compositional aid. A shot, like the anglia below doesn't really obey the rule of thirds, but by putting a slant on it, the car would be aiming more at the diagonal line and at least hinting to the rule of thirds points more.
I'm guilty of using it, but more so, when there is not a lot else to tie a shot together in terms of composition.
Also, a straight side on pan can be pretty standard and boring if taken dead square on, and central in the frame. Putting a slant on it can make it look a bit more dynamic and add interest to an otherwise standard, unremarkable shot.
I'm not saying that it should be done, however, I think that there is a time and place for it.