Like a train wreck, baby: my eyes are inexorably drawn in to the center of scene, blood flow increases to the cerebral cortex, and my brain and body respond as programmed by thousands of years of evolution coursing through veins.
Which brings me to a point: I've read some of the responses to your question, and there seems to be a conflation of questions. Be clear that I am only answering the question "Why do you watch porn?", not the question "Why do you compulsively yank your cock like a deranged chimp at the zoo when the Catholic schoolgirls stop in front of your cage?"
I watch porn to further the mood, to help engage my brain, during those times when I need it. I watch porn with my SO, as a prelude - a sort of mental lubrication - to further sexual activity; I watch porn when I masturbate to keep my mind occupied. The biggest problem I have, sexually speaking, is keeping focus: my mind wanders, especially when there is no other involved to engage more senses.
By myself, there is nothing to taste, nothing to smell, nothing to explore, nothing to touch, nothing to press my tongue against, both enjoying the sensation of my tongue, and delighting in the reaction I provoke in the other: there is just me, alone, with all my normal stressors. A mountain of debt, a lifetime of insecurity, obligations to be met, dreams to be dashed, problems to be solved: no amount of Viagra can overcome all this. There is nothing so quite disheartening as to feel the need for an orgasm, to have the time to pleasure oneself, to get started in the act, only to discover mid-stroke that I'm now thinking about my mother's impending death from brain tumors raging inside her skull.
Porn helps to engage my brain, to quell those thoughts of my existence which rob my erection of its life. Porn enables me to relax, to enjoy the sensations I'm provoking in myself, to reach that point where I am one with my cock, that ultimate Zen of existence in which each wave of pleasure moving from the head of my cock through my body defines me wholly.
I do remember, somewhat fondly, when all it would take to help this along was the mere sight of a breast. Ah, the days of our youth; soon left behind as we realize that, just like listening to rock music, you become quite accustomed to 110dB sound pressure level, and yearn for more. Until you reach that point of oversaturation, when your physical system just can't tolerate any more. Then, it's no fun: it's just oversaturation.
Sometimes, with a partner, the issue of sex becomes quite confusing; I'm not quite sure if this is a good time to approach the subject. I sense that, as I wish to fuck her, she wishes to fuck me: yet, for whatever reason, there is a gap between us, an emotional gap, perhaps, which prohibits us from just doing what bunnies do. These are the times when popping in a DVD on the big screen can do more than break the ice: it can melt the ice more certainly than any global warming threatening a glacier. Sometimes it's just enough for each of us to then masturbate, each engaged by identifcation with the onscreen antics, and yet aware of the other likewise engaged. More often, we end up entwined on the floor, the onscreen action ignored.
I also watch porn because it can provide reassurance: for whatever Freudian reason, I've always liked licking my partner's asshole. Perhaps through random bad luck, perhaps this is the way it works, the first partners I tried this with were not receptive. At all. I began to feel abnormal, ashamed of this desire which raged within me. Imagine my delight when I discovered (through porn) that, in the gamut of sexual behaviors, this was something that other people did, and enjoyed. I began to feel less ashamed, to be able to revel in the pleasure which this act brings to me (and I hope my partner), and, became more aggressively confident in asserting my normalcy in the wave of desire to bury my face between the cheeks of a pretty girl's ass, and let my tongue circle and probe her asshole.
We also, against the advice of sex therapists, learn new activities through porn, activities which not only provide for sexual release, but which increase our bond. Porn is a great teacher, and a great normalizer: it helps us both to realize that such sexual exploration is healthy and normal. It reduces the barriers which society has erected in our minds.