Why do so many people read the book AFTER they've seen the movie?

I answered another question on here about a good book for a high schooler to do a book report on. Seems everyone liked the teenage heart throb movie "Twilight" so much that now folks are buying the book to read. Am I the only one that thinks this is backwards? Do people only read books after they've seen the films? And, this is certainly not a new phenomena. I recall back in my misspent youth, when the movice "Jaws" came-out, I suddenly saw people carrying around copies of the book - "Jaws".