Well, when you shine a flashlight on your face while telling a scary story, it really amps up the atmosphere. First of all, the contrast between the light and the surrounding darkness is intensified. This draws the listener's attention directly to your face, and any small movement or expression is highlighted. For example, if you suddenly widen your eyes or make a strange grimace, it becomes much more dramatic under the flashlight. Also, the way the light might hit certain parts of your face and leave other parts in shadow can give the impression of something lurking in the darkness just behind you. It's a simple but very effective trick to make the scary story more immersive.
The flashlight on the face can create eerie shadows that distort your features, adding an instant spooky touch. It makes your face look strange and unrecognizable in parts, which is very effective for scaring the listener.