Originally Posted by pineapple stu
Provisional drivers aren't used to noticing all they're supposed to notice. Like a light that's just gone red up ahead or a car which has stopped ahead of you to turn. That can easily cause a nasty crash, which the full licence driver is more likely to spot and warn the provisional driver about. They can also, I suppose, teach them their driving - improve hill starts, stalling the car, etc - things which could also lead to crashes. Not perfect, but better than letting provisional drivers out on their own.