Porn is about control. A person tells themselves a false story about their sexuality (even if they can't articulate it or form it into words) that goes something like "I am not satisfied with my current sexual experiences, but if I find the right type of images/videos/fetishes and watch them, I will be satisfied" -- The person fulfills their sexual desires through porn use, but then it doesn't satisfy and they are back for more the next time.
You can't control a real person as easily as porn and so the user continually goes back to porn. Over time the user prefers porn to the other person in the relationship (usually a woman). Women especially, will try to compensate for the gap by comparing themselves to the women in the porn videos, but to no avail and the woman feels more and more dejected.
Heavy porn use also leads to erectile dysfunction and other issues (even if not in a relationship). Some helpful info is here: http://yourbrainonporn.com/