When a writer fails to recognize or understand the movement of a story, they risk creating plot devices that are irrelevant to the advance of a story along either its plot or story line. Such plot devices, no matter how temporarily engaging, are always ultimately unfulfilling. Blatty has carefully constructed the opening of the novel so that it has a beginning to its story line intermeshed with his plot line. And the story will be acted out by a character determined to resolve what's at stake in the story. Because he has designed the prologue to accomplish those goals, he has created an engaging, dramatic opening to his story.
Every novelist considering an opening for a story faces the same issues.
How to introduce the story and its story line. Creating a plot line intermeshed with a story line. Another note, Blatty has clearly constructed a story question for this novel. To get the answer, one has to read to the end of the novel. Blatty also takes great care to make sure each moment of his prologue is written to be dramatic, to create an anticipation of an outcome.