When speaking of Saw and Hostel, I'm assuming you're trying to gauge why people would want to watch movies with belligerent depictions of torture and the such. There are much worse out there, the Guinea Pig series being the most infamous of them (which you can find discussion on in the J-Horror forum)
Horror (or exploitation in particular, if you'd like) can also serve as a personal litmus test. Just like some people like to test their physical limits, whether that be physical fitness or pain, horror movies can test the boundries of the psyche. How much can you take before you have to look away?
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