Whether it's the thing your parking pass goes on in your car, or the information on your computer, the name derives from something from the time of horse and buggies. It was a sheet of wood that would block the mud that would be "dashed up" by the horses hooves.
Strange that we use the term cob when describing the beautiful webs that our spider friends construct. The only reason is because we used to call spiders "cobs" a very, very long time ago.
We are all now so used to seeing trailers before the movie begins, but that wasn't always the case. They used to be shown at the end to let people know what was coming up soon at a point when they would have a better chance of remembering it.
This make's sense, since we don't put trailers at the front of things, do we?