ki4amd,
I used to simply spray the dashboard letting the overspray hit the window. I just had a much more difficult time cleaning the windshield. The dash spray wasn’t the easiest thing to get off the window, especially lying in the position I needed to be in to get the window clean. I just figured using a piece of cardboard would help keep extra crud off the window. The cardboard also comes in handy when doing the reverse: I use the cardboard to cover the dash (after it’s nice and clean) to prevent the window cleaner from misting all over the dash and spotting up the clean shine. For a third purpose, I use the cardboard to cover the door windows when I spray leather or vinyl conditioner / cleaner on the door panels.