In what way does transformation play a role in stories meant to scare us?

Transformation is the conversion from one thing to another. It could be from good to bad or from bad to good. Transformations have the role of frightening us by showing that appearances deceive. You will have to be somewhat more specific with regards to what form of transformation you refer to. Red riding hood, Shaper shifter and the wolf, multiple personality disorders, etc. Sometimes things seem one thing, but the transformation reveals that it is another and the purpose of this is to surprise us.