Knowing that we humans have imaginary faculties available, why do we still need/rather have physical images to satisfy our wants/needs?

The question came up this evening while watching a talk show where child pornography and child molesting is a subject of debate.
They talked, besides the upcoming court case of a big child abuse case in Amsterdam, about the fact that thousands and thousands of photos and movies wander about the internet.
I wondered, why do people who have a want or a need for certain images, want/need to have physical photos be they real physical or on a screen of any kind to get satisfied?
(Like I said, the question arose because of the child porn case, but I am talking about any kind of image that people want/need.)
Is it (should it be/could it be) not enough for us humans to work with our imaginary skills/fantasy (to satisfy our needs)?