The idea of scripting is to obviously go into the start of the game knowing pretty much what you will run so you have practiced it and know what to expect. The other side is you are getting a read on how the D will react to formations and different looks you present which gives you some info in your pocket for later on. People are of the opinion that plays are "sent down" to the field which is not true from my experience. The people upstairs have the view so they can tell them below what each position group is doing - and this is in combination with the coaches asking "what are you seeing out there" when they come off the field. Even at the lower levels of FB, the coach on the field can't see everything, like a DL stunting or one person who has tendencies or backers who are offset a step or back a couple, it goes on and on.
You see coaches looking at their cheat sheet all the time. Each coach goes in to a game wanting to run some basic plays based on what they see on film. Then there is a list to take advantage of certain sets or opposing players. There are also plays designed for specific down and distance. Trying to remember them all on the field with a few seconds to think is brutal. Something everyone should try, once.